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Specialists in Satellite Imagery and GeoSpatial Solutions

21

Specialists in Satellite Imagery and GeoSpatial Solutions

ALOS PRISM DSM

5m cell size

3-5m Z accuracy

WorldView-1, -2 DSM

1m cell size

0.5-0.7m Z accuracy

DIGITAL ELEVATION MODELS FROM SATELLITE IMAGERY

A note from our CEO, Bob WalkerEarlier in 2010 Geoimage, in association with DigitalGlobe, hosted several popular road shows around Australia introducing the new WorldView-2 satellite. Six months later, we continue to be amazed at the clarity of the 0.5m colour resolution, the speed at which this satellite captures imagery which is then available for download within hours, and of course, the unprecedented remote sensing advantages of now having 8 bands of multispectral imagery available. WorldView-2 promised a lot at the time of our road shows, and has since lived up to every expectation.

Since the road shows, Geoimage has processed a large amount of WorldView-2 imagery from around the globe. In addition to the clarity, speed of capture and the 8 bands, we have also been impressed by the geolocational accuracy. Orthoimages produced using a good Digital Elevation Model usually only require one accurate ground control point to be shifted into the correct location with a sub-pixel accuracy. This same accuracy is also achieved in the production of Digital Surface Models from WorldView-1 and WorldView-2 stereo pairs. Geoimage is now routinely achieving vertical accuracies of 0.5m – 0.7m (LE90), with only one or two accurate ground control points required. This speed of capture and the amazing accuracies obtained are good news for clients requiring detailed terrain information over anywhere in the world.

This brochure presents several case histories of the excellent results that we are now seeing from the WorldView DSMs, including how they have been used in conjunction with other satellite data sources. I am sure that you will be as impressed with the results that have been achieved as I am, and if you have any queries, please contact me or one of our friendly team in either our Brisbane, Perth or Sydney offices.

Best wishes to you all, Bob Walker

DEMS FROM SATELLITE IMAGERYIntroductIonDigital Elevation Models (DEMs) are an integral part of any GeoSpatial Analysis. They are required both for the description of the three dimensional surface and to orthorectify the imagery which is used as a backdrop and to provide derived information for modelling purposes.

The scale and level of detail required in an individual project will determine the spatial resolution and accuracy of the DEM required. For very large scale projects covering tens of thousands of square kilometres, the SRTM and GDEM world-wide DEMs are probably most appropriate and details of these are discussed below. At the very detailed level, the demand for DEMs is met by LIDAR surveys from aerial platforms which typically have spatial resolutions of a metre and accuracies better than 15cm. Such surveys may be very expensive because of locational charges or may not be available due to flying restrictions. In such circumstances, DEMs from satellite platforms which have lower vertical accuracies but cover much large area and are less expensive per sq km, may be the best option.

dEFInItIonSThe term DEM is a generic term that includes two distinct topographic models and it is important to recognise the distinction as it will affect how useful the model is for any application. A Digital Surface Model (DSM) is an elevation model of surface reflectance features and includes the heights of cultural features such as buildings, road and vegetation as well as bare earth. It is this model that is produced from a pair of stereo images and is used to orthorectify satellite imagery. The Digital Terrain Model (DTM) is a bare-earth model in which all the cultural features have been removed (from a DSM) and is the model that would be used for example for flood modelling.

In this booklet, we have used the terms CE90 and LE90 to refer to the accuracies of location. CE90 refers to horizontal accuracy and means that objects are located within that accuracy 90% of the time while LE90 is a similar measure in height.

SuMMArY oF dSMs And AccurAcIESThere is now a satellite sourced DEM available for most scales from the truly continental down to the very detailed project scale where accuracies of a metre are required. The SRTM and GDEM fill the continental role and are freely available however it helps to understand their limitations. In the case of SRTM it should be remembered that large areas in very steep terrains had no-data voids in the original data and these now have interpolated fills. In the case of the GDEM, the currently available data is only the first release and contains artifacts that I am sure in subsequent releases of the data will be fixed.

The SPOT 5 payload includes the HRS Imaging instrument developed by Astrium for DEM generation. This instrument uses cameras looking at 20deg fore and aft to image stereo pairs over a surface area of 600km along track and 120km across track centered on the satellite track. The spatial resolution of the instrument is 10m and sampling cross-track is 10m and along track is 5m. SPOT IMAGE offers two DEM products from the HRS data which are referred to as SPOT DEM and Reference3D. Full descriptions of these products are available at http://www.spotimage.fr/html/_167_224_807_.php.

For detailed DEM work and depending on the scale of the project, Geoimage recommends DEMs produced from either the WorldView satellites or the ALOS PRISM. The details of the DEMs produced from these satellites are summarised below and are described more fully in the following pages.

PotEntIAL GEoSPAtIAL ASPEctS In the case of WV-2 and GeoEye-1, multispectral imagery is collected at the same time as the panchromatic imagery that is used for the DSM generation. This multispectral data is usually bundled with the pan and its use in complementing the DSM and providing spatial information layers should not be overlooked. Two such applications are discussed on Pages 10 and 11 and Geoimage’s GeoSpatial Team would be happy to discuss other applications with you to ensure that the maximum use is made of any data purchase.

Front coVErThe images on the front cover are a WV-1 derived DSM on the top with a 2m contour and an ALOS derived DSM on the bottom with a 5m contour. The area is adjacent to the Superpit at Kalgoorlie and includes the tailing dams to the east of the town. As reported on page 9 of this brochure the PRISM DSM has an error of 2m LE90 compared to the WV-1 DSM. Note the high frequency “noise” in the WV-1 DSM corresponding to healthy trees. © DigitalGlobe 2009

Source Spatial resolution(m)

Accuracy (m)

Availability

SRTM 90 6-8 Land areas between latitudes 60N and 56S

GDEM (ASTER) 30 Variable Most land areas

SPOT HRS 20-30 8 From archive

ALOS PRISM 5 3-5 From archive

IKONOS 2 1-2 New Capture

WorldView-1, WorldView-2, GeoEye-1

1 0.5–1.0 New Capture

WorldView-1 and -2 dSMs ALoS PrISM dSMs

ADVANTAGES

1m cell size 5m cell size

Accuracy in E and N better than 5m CE90 without GCPs

Accuracy in E and N dependent on ground control

Accuracy better than 1m CE90 with GCPs Accuracy in E and N without ground control about 15m

Z accuracy 0.5 to 0.7m LE90 Z accuracy better than 5m LE90 and usually 2-4m.

Available by new capture request Usually available from archive

Low off-nadir capture of the NADIR gives a very accurate orthoimage

DISADVANTAGES

Not available from archive Unable to program new capture

15% cloud seen as acceptable capture DSM not a DTM although may give the elevation of the ground with sparse vegetation.

Digital Surface Model not a Digital Terrain Model DSM may be noisy in flat terrain.

May have high off-nadir capture angles and this may result in smearing in the orthoimage.

Comparing the accuracy and availability of stereo satellite imagery to SRTM and GDEM

SrtMThe Shuttle Radar Topography Mission (SRTM) collected Interferometric Synthetic Aperture Radar (IFSAR) data over land between 60 degrees North and 56 degrees South latitudes in February 2000 from the Space Shuttle Endeavour. The mission was co-sponsored by the National Aeronautics and Space Administration (NASA) and the National GeoSpatial-Intelligence Agency (NGA). NASA’s Jet Propulsion Laboratory (JPL) processed the raw C-band radar data into a preliminary, partially finished version of terrain elevations and related products. These were subsequently finished by NGA contractors and conform to the NGA SRTM Data Products Specification and the NGA Digital Terrain Elevation Data (DTED®) Specification.

The SRTM DEM is a uniform matrix of elevation values with a resolution of 3 arc second (approximately 90 m at the Equator) and available in 5° by 5° tiles in GeoTiff format. The horizontal datum is the World Geodetic System 1984 (WGS 84) and the vertical datum is mean sea level as determined by the WGS84 Earth Gravitational Model (EGM 96) geoid. The elevation values represent the reflective surface (DSM), which may be vegetation, man-made features or bare earth. A 1 arc second (30m) grid of the Continental US is also available.

SrtM Version 2 was extensively edited by NGA contractors. Spikes and wells in the data were detected and voided out if they exceed 100m compared to surrounding elevations. Small voids (16 contiguous posts or less) were filled by interpolation of surrounding elevations. Large voids which are left in the data may be due to shadows, layover, poor reflective properties of the Earth’s surface, or excessive noise in the data. Water bodies are depicted in the SRTM DTED® and ocean elevations are set to 0 metres. Lakes of 600m or more in length are flattened and set to a constant elevation. Rivers that exceed 183m in width are delineated and monotonically stepped down in height. Islands are depicted if they have a major axis exceeding 300m or the relief is greater than 15m. The data is currently distributed free of charge by USGS and is available for download from the National Map Seamless Data Distribution System, or the USGS ftp site.

A reprocessed version of the Continental 90m SRTM DEM (Version 4) is available from the CGIAR-CSI GeoPortal . Dr. Andy Jarvis and Edward Guevara of the CIAT Agroecosystems Resilience project, Dr. Hannes Isaak Reuter (JRC-IES-LMNH) and Dr. Andy Nelson (JRC-IES-GEM) have further processed the original DEMs to fill in the no-data voids. This involved the production of vector contours and points, and the re-interpolation of these derived contours back into the raster DEM. These interpolated DEM values are then used to fill in the original no-data holes within the SRTM data. These processes were implemented using Arc/Info and an AML script. The DEM files have been mosaiced into a seamless near-global coverage (up to 60 degrees north and south), and are available for download as 5 degree x 5 degree tiles, in geographic coordinate system - WGS84 datum. These files are available for download in Arc-Info ASCII format and GeoTiff. In addition, a binary Data Mask file is available for download, allowing users to identify the areas within each DEM which has been interpolated. Note: These data are provided free for non-commercial use however commercial users are asked for a contribution towards the development of the product. http://srtm.csi.cgiar.org/

The SRTM has been compared to other independent sources of elevation data to evaluate the accuracy of the SRTM data. The absolute height accuracy appears to be significantly better than the 16 metre (90% confidence) specification for the mission, and the horizontal accuracy meets the corresponding 20 metre specification. The results of an extensive ground campaign conducted by NGA and NASA to collect ground-truth to validate the SRTM result are summarised in the Table below with all quantities representing 90% errors in metres.

continent Africa Australia Eurasia Islands n. America S. AmericaHorizontal Accuracy 11.9 7.2 8.8 9.0 12.6 9.0

Absolute Vertical 5.6 6.0 6.2 8.0 9.0 6.2

Relative Vertical 9.8 4.7 8.7 6.2 7.0 5.5

GdEMThe Ministry of Economy, Trade, and Industry (METI) of Japan and the United States National Aeronautics and Space Administration (NASA) jointly released Version 1 of the Advanced Spaceborne Thermal Emission and Reflection Radiometre (ASTER) Global Digital Elevation Model (GDEM) on June 29, 2009. Previously, METI and NASA had announced their intent to contribute the ASTER GDEM to the Global Earth Observation System of Systems (GEOSS).

Consequently, the ASTER GDEM is available at no charge to users worldwide via electronic download from the Earth Remote Sensing Data Analysis Center (ERSDAC) of Japan and from NASA’s Land Processes Distributed Active Archive Center (LP DAAC).

The methodology used to produce the ASTER GDEM involved automated processing of the entire 1.5-million-scene ASTER archive, including stereo-correlation to produce 1,264,118 individual scene-based ASTER DEMs, cloud masking to remove cloudy pixels, stacking all cloud-screened DEMs, removing residual bad values and outliers, averaging selected data to create final pixel values, and then correcting residual anomalies before partitioning the data into 1° by 1° tiles. Each GDEM file is accompanied by a Quality Assessment file, either giving the number of ASTER scenes used to calculate a pixel’s value, or indicating the source of external DEM data used to fill the ASTER voids.It took approximately one year to complete production of the beta version of the ASTER GDEM using a fully automated approach.

The ASTER GDEM covers land surfaces between 83°N and 83°S and is composed of 22,600 1° by 1° tiles. Tiles that contain at least 0.01% land area are included. The ASTER GDEM is in GeoTIFF format with geographic lat/long coordinates and a 1 arc-second (30m) grid of elevation postings. It is referenced to the WGS84/EGM96 geoid. Pre-production estimated accuracies for this global product were 20 metres at 95 % confidence for vertical data and 30 metres at 95% confidence for horizontal data.

Initial studies to validate and characterize the ASTER GDEM confirm that pre-production accuracy estimates are generally achieved for most of the global land surface, although results do vary and true accuracies do not meet pre-production estimates for some areas. In addition, Version 1 of the ASTER GDEM does contain certain residual anomalies and artifacts that affect the accuracy of the product and may be impediments to effective utilization for certain applications. Consequently, METI and NASA acknowledge that Version 1 of the ASTER GDEM should be viewed as “experimental” or “research grade.” Nevertheless, they are confident that the ASTER GDEM represents an important contribution to the global earth observation community. It is suggested that prior to using the data, users obtain a copy of the ASTER Global DEM Validation - Summary Report.http://www.gdem.aster.ersdac.or.jp/ASTER_GDEM_Validation_Summary_Report

Coloured SRTM image showing the latitude limits of 60°N to 56°S.

Greyscale GDEM image showing the latitude limits of 83°N to 83°S.

(A) SRTM image from an area in central Asia and (B) GDEM for the same area. The artifacts in the GDEM are presumably due to areas of inaccurate masking of cloud and shadow and can be edited out if they are not too frequent. GDEM is of particular importance in areas outside the latitude limits of SRTM i.e. above 60 deg N and below 56 deg south.

A

B

Most of the current push-broom satellite sensors have a stereo capability whether it is in-track stereo such as the ALOS PRISM and ASTER, cross-track stereo such as the SPOT satellites, or a combination of in-track and cross-track stereo as is the case with the agile Very High Resolution (VHR) sensors. The methodology for creating the DEM is independent of the sensor and involves the following processing steps:

• Selection of several accurate GCPs or tie points in the stereo pairs,

• Computation of a 3-D stereo model based on the sensor parametres and refined using the GCPs,

• Projection of the original imagery as epipolar images in which the left and right images have a common orientation and the height distortion is maximised in an east-west direction,

• autocorrelation between the stereo pairs on a pixel by pixel basis and computation of XYZ cartographic coordinates from the elevation parallaxes in a regular grid spacing, and

• reprojection of the epipolar dem back to real world coordinates.

The stereo extracted elevation model is a Digital Surface Model (DSM) and includes the height of natural and man made features such as vegetation and buildings. The quality and accuracy of the DSM will be dependent on many factors including -

Pixel size of the imagery. The finer the resolution of the sensor obviously the higher resolution and detail of the resultant DEM.

Base to height ratio of the stereo pair. In the case of aerial photos, the base to height (B/H) ratio is used to assess the potential accuracy of a stereo pair to generate DEMs. The ratio is defined by the separation of the pair divided by the height of the sensor. In the case of satellite sensors with fixed telescopes, a similar ratio can be used and gives ratios of 0.6 for ASTER, 0.85 for SPOT HRS and for ALOS PRISM 1.0 for forward - backward view and 0.5 for the individual off-nadir - nadir view. Hasegawa et al (http://www.isprs.org/proceedings/XXXIII/congress/part4/356_XXXIII-part4.pdf) have shown that DEM accuracy increases rapidly in proportion to the B/H ratio up to 0.5 and gradually decreased at ratios higher than 1.0. They conclude that a B/H ratio between 0.5 and 1.0 is best for DEM creation.

In the case of the VHR satellites with their pointable telescopes, the B/H ratio is not appropriate as a measure of the effectiveness of the stereo pair for DEM generation. In such cases, three angular measures of convergent stereo imaging geometry: the convergence angle, the asymmetry angle, and the bisector elevation angle (BIE) are used. These measure the geometrical relationship between two rays that intersect at a common ground point, one from the fore image and one from the aft image as shown in the diagram.

The most important of the three stereo angles is the convergence and is the angle between the two rays in the convergence or epipolar plane. An angle between 30 and 60 degrees is ideal. Asymmetry describes the apparent offset

from the centre view that a stereo pair has. For instance, a stereo pair with an asymmetry of 0° will have parallax due to elevations that appear equivalent in the left and right images. An asymmetrical collection is preferred as it gives a different look angle to discern ground features more accurately but should be under 20 deg. The BIE angle is the angle between the horizontal plane and the epipolar plane and defines the amount of parallax that will appear in the vertical direction after alignment. The angle should be between 60 and 90 degrees.

radiometric quality of the imagery. Since the panchromatic band used in the stereo pairs is invariable a combination of the visible spectra, the imagery is subject to haze problems in tropical areas. In the case of PRISM, the imagery is characterised by significant digital compression noise due to JPEG compression for transmission from satellite to ground

Ground cover. Large areas of no texture - examples are playa lakes, ocean, rivers and lakes, area of low albedo - examples are forested areas or imagery with low sun angles, and repetitive textured areas e.g stripped paddocks, all produce problems during the image correlation.

relief in the area. Very steep area such as coastal cliffs, edges of tablelands, incised river valleys, benches in open pits, etc, will often be missing in one of the stereo pair images because the slope is steeper than the angle of capture. Steep area will also contribute to correlation problems as the features of the landscape which are being correlated will look different.

Quality of the ground control points. GEOIMAGE provides a guide to its clients on how to collect the GCPs needed for accurate control.

time difference between stereo images. For cross-track stereo pairs, the greater the time difference between the images, the higher the probability of spectral and spatial differences in the images. Changes in cloud cover, fires, vegetation , glints on water bodies, etc will all contribute to correlation problems.

dSM noISEHigh frequency, relatively low amplitude noise is commonly seen in DSMs created directly from stereo imagery by auto-correlation. It has been reported from a number of satellite sources used for DSM creation, including PRISM, ASTER, SPOT and Shuttle Topographic Mapping (SRTM). Most commonly the noise manifests as small irregularly located pits and spikes. Its presence is often reflected in lower measured absolute and relative accuracies of resultant surfaces in areas where the surfaces are flat and approach horizontal. In areas where there is more topographic relief this noise tends to become insignificant.

It is not possible to remove this noise completely. Filtering techniques generally also result in removal of high-frequency detail from the satisfactory areas of the generated surface, and/or reducing the amplitude of the noise spikes and pits, but spreading their areal extent. They are applied carefully and used sparingly. Geoimage routinely use two techniques to reduce the effects of the noise. these include preprocessing of the raw imagery to reduce compression effects and reinforce edges and in the case of PRISM DSMs producing DSMs from all stereo pairs and averaging of the results - a technique similar to stacking in seismic processing which is based on the assumption that the noise is random whereas signal is not.

SATELLITE STEREO-PAIRS

Highlighting some of the problems of stereo image correlation. SPOT 5m Stereo pair - after orthorectification. Scale approximately 1:60 000. Left image collected 31 Jan 2005 Incidence angle L24.68deg. Right image collected 11 Feb 2005 Incidence angle R30.405. © CNES 2005.

The two white areas annotated A are water bodies that have caught a sun glint on the left image and are black on the pair. All the water bodies in the image regardless of size, including a wide river, had the same glint and were much brighter than the surrounding land while in the right image they were very low albedo and darker than the surrounding land.

The paddock marked B is best see n in the right image where it is much darker than the surrounding paddocks and the change can only be attributed to a fire.

The areas marked c are wisps of cloud on the right and the associated cloud shadow on the left. Such cloud is difficult to see in such a busy image.

This image also produced correlation problems in some of the paddocks with contour plowing, which had variable visibility in the two images based on the particular aspect of the paddock.

A A

B

C

DigitalGlobe offers Basic stereo pair image products from both the WorldView-1 and Worldview-2 sensors. The imagery is collected in-track, i.e. on the same overpass, and supplied as full track-width data with a minimum capture area of 15km wide by 14km north-south. The products are ideal for DEM generation, 3D visualisation and feature extraction application and details of the product are listed below.

Although the stereo products can be ordered out of archive, the possibility of an area being available is low so most orders will require a new collection request. Timing of any such request will be depend on a multitude of circumstances however we have had some good experiences with very fast captures of WV-2 stereo in 2010. If the client only requires a DEM (and does not require a colour orthoimage product), a WV-1 capture may be a faster capture option.

With new captures, cloud and shadow are always a concern as imagery with less than 15% cloud must be purchased. In several projects where our clients have had to accept stereo captures with cloud, Geoimage has been able to offer solutions to the client based on producing a DSM product from archived stereo PRISM imagery and seamlessly merging of these DSMs.

Special promotionDigitalGlobe currently have a special promotion for stereo imagery which offers areas smaller than the normal 210 sq km and available until 31st December 2010. Conditions of this promotion are:-1. Minimum order size is 100 sq km. Normal criteria for minimum sized vector surrounds

applies.2. A 2km by 2km cloud free area based on a coordinate supplied by the client.3. Offer only open to Australasian based clients although the area of capture is not

restricted.4. Discounted stereo imagery pricing applies (Contact Geoimage for a quotation).5. The offer is based on the supply of standard scenes of Basic Level Stereo data by

DigitalGlobe and DEM generation/ortho imagery processing by Geoimage. Geoimage will then supply these products over the actual area ordered by the client.

EXAMPLES oF dSM ProcESSInGGeoimage processes WorldView stereo pairs in a combination of Socet Set and PCI OrthoEngine and have processed over 20 separate projects. The following is a summary of our overall results but with specific results based on a WV-1 stereo pair over the superpit in Kalgoorlie. The WV-1 stereo pair over Kalgoorlie was collected on 02 October 2009 and the angles of capture and three angular measures of imaging geometry are shown in the table below while the aerial extent of the pairs is shown in the figure. The elevation range in the normal surface terrain in the region is 320 to 455m with values to -254m in the Superpit.

Initial processing of this DSM was done in Socet Set using a combination of the RPC and about 10 accurate ground control points however after extensive experince with other WV stereo pairs the processing reported here is for a completely systematic processing of the stereo pair and the geometic checking of the DSM and orthoimage against +50 surveyed ground control points.

THE DSM was generated at one metre cell size using NGATE. Correlation between the pair was good and there was approximately 95% correlation at the highest confidence level. The DSM was then used to systematically orthorectify the most vertically captured image using just the DigitalGlobe supplied ephemeris information. The location of the ground control points were checked on this orthoimage and the results are summarised in the table below.

In summary these results show that after a block adjustment in the orthoimage and the DSM of 1.28m in Easting and -2.89m in Northing, the location of an object in the orthoimage will be within 0.4m at CE90 and the elevation will be within 0.56m LE90. Similar results have been obtained from several WV-2 stereo projects where we have had accurate surveyed ground control and/or accurate vector information from the client.

It should be remembered however that the DSM gives the height of surface features such as vegetation, buildings etc and not the ground surface. In the Kalgoorlie area, the vegetation is mainly low scrub however in the drainage swales, there are often tall trees which do show in the DSM (see also the cover graphic).

Kalgoorlie WV-1 stereo pair collected 02 october 2009. The red image was collected from an azimuth of 348 deg and the cyan image from an azimuth of 283 deg. The yellow box defines a 15km by 14km area which is usually the minimum order area for a new WorldView capture. © DigitalGlobe 2009

Minimum orderable Area 210km2 (15km x 14km)

Product Framing Full width

Pan strip width (km, approximate at nadir)

16.4 (WV-2)

17.7 (WV-1)

Absolute Geolocational Accuracy (Nadir)

Geometrically raw. With supplied image support data imagery can be processed to

6.5m CE90 (WV-1, WV-2) or 23m CE90 (QB) at nadir, excluding terrain effects.

Product Options Pan, 4-band, pan + 4-band bundle

Number of bits/pixel for

deliverable image

8 or 16 (suggested)

Output pixel spacing As collected; no worse than 75 cm

Overlap of AOI 100%

Convergence angle C 30–60

Bisector elevation angle (BIE) 60–90

Asymmetry < 20 degrees

WORLDVIEW-1 AND -2 STEREO PAIRS

A B C

Detailed view of the Kalgoorlie WV-1 orthoimage (A) showing the location of tall eucalypt trees in the swale area which show up in the raw DSM (B) which has a range of elevations from 388 to 410m. Geoimage has used a semi-automated method to edit out the vegetation in image (c). The black contours in B and C are at 2m intervals. © DigitalGlobe 2009

Satellite Azimuth

off-nadir angle

Image 1 348 35.5

Image 2 283 15.2

Target

Convergence Angle 31.79 30 – 60 degrees

Asymmetry Angle 16.59 < 20 degrees

Bisector Elevation Angle (BIE) 69.7 60 – 90 degrees

Stereo Angles for Kalgoorlie WV-1 stereo pair

Easting northing Elevation

mean 1.284906 -2.88691 -9.10648

Standard deviation 0.213312 0.251523 0.342912

ce90 absolute 1.331957 2.916405 Le90 absolute 9.123933

ce90 relative 0.350898 0.413755 Le90 relative 0.56409

Differences in the Easting and Northing of the GCPs between the surveyed coordinates and their location in the systematically orthorectified image. The difference in the elevations is after the adjustment of the systematic DSM by 1.28m in Easting and -2.89m in Northing.

cHArActErIStIcS oF WV-1 And -2 BASIc StErEo PAIrS

VHR DSM GENERATION TEST CASE - CAPELLAIntroductIonAn area of approximately 460 sq km was targeted for stereo WorldView-2 capture in the Capella area of Central Queensland. The stereo capture took place on 18th July 2010 within a week of the order confirmation and the capture angles were ideal for DSM generation.

Unfortunately there was cloud over the area of interest during the capture and as this was under the cloud threshold of 15% and was not over the designated 2km by 2km cloud free area, the image had to be purchased. Geoimage was able to offer the client a solution based on infilling the cloud-affected areas with an ALOS PRISM DSM generated from a PRISM triplet that had been collected on 29 July 2008.

WV-2 dSM ProcESSInGThe DSM was generated in Socet Set using NGATE at a 1m cell size and using system parametres and tie-points but without any ground control. As was to be expected, the DSM had approximately 20% null areas cause by non-correlation over the cloud and shadow areas. The DSM was used to systematically orthorectify the most vertical WV-2 image and two accurately surveyed ground control points were identified on this image. No block shift in easting or northing of the DSM/orthoimage was necessary as the ground control were within a half pixel of their surveyed location. A block shift of 1.6m was necessary in the DSM to adjust it to the ground control.

PrISM dSM ProcESSInGThe ALOS DSM was generated from the PRISM triplet in PCI OrthoEngine using ground control from the WV-2 orthoimage and the WV-2 DSM for height control. The DSM was actually an average of six individual DSMs generated from the B-N, N-B, N-F, F-N, F-B and B-F pairs. After production at a cell size of 5m, the DSM was resampled to 1m so as to integrate better with the WV-2 DSM.

comparison of WV-2 and ALoS PrISM dSMsSince the ALOS PRISM imagery was controlled using the systematic corrected WV-2 DSM and orthoimagery, a comparison of the two DSMs should reveal the accuracies that might be expected of an ALOS PRISM DSM in a low slope terrain. A histogram plot of the difference between the DSMs gives a positively skewed histogram. When the DSM images are compared in detail and with the orthimage,

it is obvious that the ALOS PRISM DSM is either averaging the vegetation height or giving the ground surface below the vegetation depending on the amount of the vegetation present. If we exclude the vegetated areas using an NDVI from the DSM difference image, we get a normal distribution for the histogram.

For the non-vegetated areas, the difference between the WV-2 DSM and the PRISM DSM has a mean of 0.094, a standard deviation of 1.685 and an LE90 value of 2.77m. This LE90 value is considered to be a better measure of the accuracy of the ALOS DSM than that which includes the vegetated areas.

WV-2 - PrISM coMBInEd dSM Because of the skewed relationship between the two DSMs caused by the vegetation, it was not a simple matter to join the two grids. A technique involving the local area matching of the two grids based on a moving statistical window was used and resulted in a seamless integration of the two. This is shown for a small subset of the area in the graphics below.

Natural colour image of the WV-2 stereo and the initial well correlated DSM prepared from the stereo pair showing the holes in the DSM caused by the cloud and cloud shadow. © DigitalGlobe 2010

Integration of the WV-2 and PRISM DSMs. (A) WV-2 DSM with null areas. (B) WV-2 DSM infilled with raw PRISM DSM. (c) WV-2 DSM with local area matched PRISM DSM. (d) WV-2 DSM and matched PRISM DSM with averaging over joins.

Coloured ALOS PRISM DSM over a 35km by 35km area and with the WV-2 DSM generation outlined in black.

(A) Positively skewed histogram of the difference between the WV-2 and PRISM DSMs. (B) Normal distribution when vegetation is excluded.

ALOS PRISM DSM compared to the WV-2 DSM.

Satellite Azimuth

off-nadir angle

Image 1 28.2 19.9

Image 2 174.4 16

Target

Convergence Angle 34.3 30 – 60 degrees

Asymmetry Angle 2.05 < 20 degrees

Bisector Elevation Angle (BIE) 84.31 60 – 90 degrees

Stereo Angles for Capella WV-1 stereo pair

A B

A

C

B

D

VHR DSM GENERATION TEST CASE - NORTHERN BOWEN BASINIntroductIonAn area of approximately 190 sq km was targeted for stereo WorldView-2 capture in the Northern Bowen Basin. The area was captured several time in July 2010 however all scenes had cloud and the first cloud free capture was on 3rd August 2010. This capture was initially rejected because of the high off-nadir angles however it was decided to process the pair to check what results could be obtained from such angles. The initial processing was so successful, the client was happy to proceed with the purchase and processing of the imagery.

WV-2 dSM ProcESSInGThe DSM was generated in Socet Set using NGATE at a 1m cell size and using system parametres and tie-points but without any ground control. The images correlated well with the main areas of poor correlation relating to water filled old mine workings. These areas were edited out and filled with estimated water levels. The DSM was used to systematically orthorectify the most vertical WV-2 image and this orthoimage was overlain with road and other topographic vectors supplied by the client. A block shift of 4m in easting and 2m in northing was made to the orthoimage and this resulted in a perfect correlation between the vectors and the orthoimage throughout the area.

The client was able to provide the surveyed location of 36 lease corners with elevations. These points could not be identified on the imagery however their heights could be used to check the accuracy of the systematic generated DSM. After the original raw DSM was block shifted in E and N, the heights of the corners in the DSM were subtracted from the surveyed RLs. It was immediately obvious that three of the points had incorrect RLs and after exclusion of these points, the 33 remaining points had a mean difference of -0.07m, a standard deviation of 0.41m and an LE90 error of 0.69m. No adjustment or block-shift was required to the WV-2 DSM.

concLuSIonThe accuracy of the E and N location of the final orthoimage coupled with the accuracy of the DSM lead to the conclusion that in relatively low slope areas, stereo pairs with low BIE angles or high off-nadir angles, do produce good DSMs. The only possible drawback of the imagery is that at such angles, there will probably be some smearing of the imagery during the orthorectification and this may affect spectral processing of the multispectral imagery.

Natural colour WV-2 orthoimage over the full extent of the stereo capture showing the location of the client supplied vectors used to adjust the location of the imagery.

Enhanced natural colour 0.5m resolution WV-2 pan sharpened orthoimage after block adjustment showing the perfect correlation between the client supplied vectors and the orthoimage.

Coloured DSM of the area with some vertical shading showing the location of the client supplied lease corner location used to check the accuracy of the DSM.

Enhanced natural colour 0.5m resolution WV-2 pan sharpened orthoimage over a section of the old mine workings with a 2m DSM contour.

Satellite Azimuth

off-nadir angle

Image 1 309.5 30.5

Image 2 232.4 38.7

Target

Convergence Angle 41.99 30 – 60 degrees

Asymmetry Angle 7.34 < 20 degrees

Bisector Elevation Angle (BIE) 61.57 60 – 90 degrees

Stereo Angles for the WV-2 stereo pair

© DigitalGlobe 2010

Mode 1

Mode 2

IntroductIonThe Panchromatic Remote-sensing Instrument for Stereo Mapping (PRISM) is a panchromatic radiometre with 2.5m spatial resolution launched aboard the JAXA ALOS satellite on January 24, 2006. PRISM has three independent optical systems for viewing nadir, forward and backward and producing along track stereo. Each telescope consists of three mirrors and several CCD detectors for push-broom scanning. The nadir-viewing telescope covers a width of 70km while the forward and backward telescopes cover 35km each. The forward and backward telescopes are inclined +24 and -24 degrees from nadir to realize a base-to-height ratio of 1.0. PRISM’s wide field of view (FOV) provides three fully overlapped stereo (triplet) images of a 35km width without mechanical scanning or yaw steering of the satellite.

The PRISM sensor has several modes of operation of which Mode 1 - Triplet observation mode using Forward (F), Nadir (N), and Backward (B) views (Swath width of 35km) and Mode 2 - Nadir (70km) + Backward (35km) are the main ones used. In Mode 2, the backward view can be either side of the satellite track.

Since the PRISM sensor began routine collection of imagery in May 2006, JAXA has adopted a systematic observation strategy to provide a global archive of data. Unfortunately this does not allow user requests for coverage of specific areas, however for most of the world’s land masses archive coverage does exist.

dSM cHArActErIStIcS And AccurAcIESGeoimage routinely produces DSMs with a 5m cell size from the PRISM triplets using PCI OrthoEngine. E and N control on the processing may be from surveyed points provided by the client, from accurate VHR imagery of the same area, or if no control is available then we rely on the Landsat 7 ETM+ orthoimages available from EROS. Similarly Z control can be from client supplied ground control and if this is not available then we use the SRTM DEM.

It is generally found that the combination of the F-N and B-N stereo pairs produce the best accuracies while the F-B pair produces reasonable results in low slope areas but as the slope increase has more blunders and lower accuracies presumably due to the large differences in the view angles. Geoimage has developed special methodologies including special filters that reduce the inherent noise in the PRISM DSMs and is confident that we can achieve LE90 accuracies of 3-5m where there is accurate ground control and similar relative accuracies where we have to use the SRTM DEM for Z control.

The following test case studies are examples of where Geoimage have been able to accurately document the DSM accuracies that can be achieved using PRISM triplets.

Archive of ALOS PRISM triplets with less than two percent cloud from start of collection until April 29, 2010.

test case 1. reko diq deposit, Pakistan.

top PRISM DEM for the project area. Bottom PRISM DEM with 5m contours compared to SRTM DEM.

A DSM generation project was undertaken over the Reko Diq Deposit for Barrick Gold to assist pre-feasibility planning for mine infrastructure. The work involved 2 date strips of 2 PRISM triplets which were controlled with surveyed ground control provided by the client. The DSM was originally created at 5m however because of the noise the product was supplied at 10m cell size. Since this project, Geoimage has improved its noise reduction techniques and now supplies all PRISM DSMs at 5m.

The following is taken from a report prepared by the client-To test the vertical accuracy of the model, the PRISM DEM was compared with drill hole collar elevations. Results are shown in Figure 3 below: for 534 drill holes, out of 537 drill holes included, or 99.3% of the drill holes, elevations varied from the PRISM DEM by less than 5m. The maximum variation between the model and the drill collars was 35m, and the mean difference was 0.13m. The two collar RLs which varied from the DEM model by 30m or more require validation. Note that the drill collar heights were not used in the image orthorectification process, as they did not present features which could be recognised in the satellite images.

PRISM DSMS

Difference between the DEM generated from PRISM stereoscopic imagery, and drill hole collar elevations.

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COLOUR LEGEND

test case 2. Kalgoorlie.An ALOS PRISM DSM was generated for the general area of the Kalgoorlie Superpit using a triplet collected in Januray 2008. The PRISM scene completely covered the area of the DSM that was generated from the WorldView-1 stereo pair collected on 02 October 2009 (see Page 5). The availability of the very accurate WV-1 DSM was considered ideal to test what DSM accuracies could be achieved from a PRISM triplet without any ground control in a relatively low slope environment.

test case 3. YemenAn area of approximately 210 sq km was targeted for stereo WorldView-2 capture over the Suwar nickel-copper-cobalt project in the north-western part of the Republic of Yemen. It was recognised that the area would be very challenging as the elevations in the area ranged from 1000m to 3300m on the SRTM data. DigitalGlobe collected the WV-2 stereo pair on March 15, 2010 and the image quality was excellent with no cloud. The capture angles were from 314 deg with an off-nadir angle of 12.7 deg and from 209 deg with an off-nadir angle of 33.7 deg.

The PRISM DSM was generated using a Landsat 7 ETM+ Pan scene for E and N control and the SRTM for Z control and was produced at a cell size of 5m. An initial visual comparison of the PRISM and WV-2 DSMs showed major difference over the Superpit and the tailings areas and this is to be expected with the difference in dates of the imagery. When these areas are excluded from the comparison, the difference between the DSMs has a mean of 3.6m and a standard deviation of 1.195m. The 3.6m difference is believed to be mainly a result of the use of the SRTM control and after adjusting the PRISM DSM by the 3.6m, the error in the DSM was 2.0m at LE90.

The DSM comparison also reinforced the observation seen at Capella (Page 6) that while the WV-1 DSM picked up isolated and clumps of trees, the ALOS DSM either saw through the vegetation or averaged it out over the 5m cell size.

(A) ALOS PRISM 5m DSM. (B) WV-1 1m DSM showing the high frequency “noise” corresponding to both single trees and groups of trees.

(A) ALOS PRISM 5m DSM - area of WV-1 DSM outlined in red. (B) ALOS PRISM 2.5m resolution nadir orthoimage. © JAXA/RESTEC

Initial ground control supplied by the client was not sufficiently accurate and so the DSM was generated from the WV-2 stereo pair in Socet Set with no ground control and at a pixel spacing of 1m. The DSM produced showed approximately 53% well correlated points with several large areas which did not correlate well at all. It was obvious from the results that the steep slopes in the area and the large off-nadir angle of the north looking image were combining to give poor correlation predominantly in the north and east facing slopes and it was decided to obtain an ALOS PRISM triplet over the area and use the DSM generated from this to infill the holes in the WV-2 DSM. It was considered that with a combination of the F-N and N-B stereo pairs that there would not be any missing areas.

The Z accuracy of the DSM was checked against 376 gravity stations for which the client was able to provide accurate X, Y and Z locations. These points could not be accurately located on the imagery to use as ground control points but were available to check the Z accuracy at the points. Approximately 300 of the points fell on the well correlated DSM and the differences between the measured elevation and the DSM value had a mean of 0.02, a standard deviation of 2.4 and an LE90 of 3.9m. This result was considered very good considering the location of the DSM was probably only accurate to about 4m.

RESTEC/JAXA kindly provided Geoimage with a PRISM triplet collected on 26 May 2008, over the same area as the WV-2 stereo pair. A 5m DSM was produced from the triplet using a combination of the F-N and N-B stereo pairs with the X, Y and Z control based on the systematically produced WV-2 orthoimages and DSM. After deleteing several areas of bad correlation in the WV-2 DSM, the difference between the WV-2 and PRISM DSMs gave a mean of zero, an SD of 1m and an LE90 of 1.7m. These statistics appears almost too good however it must be remembered that this did not include the steepest terrains where the WV-2 stereo did not correlate. A combine WV-2 and ALOS DSMs was used to orthorectify the most vertical WV-2 image.

(A) Initial WV-2 DSM with null areas being areas of poor correlation during the DSM generation in Socet Set. (B) Composite image with well correlated WV-2 DSM in greyscale, Green areas are areas not seen from the North looking image, and Red areas are terrain with slopes over 45deg in any direction. (c) ALOS PRISM 5m DSM. (d) WV-2 natural colour Orthoimage. © DigitalGlobe 2010

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Linear InfrastructureThe acquisition of terrain data is a mission critical component of any infrastructure project. From the preliminary investigations, right though to construction, it is paramount that the project team has access to appropriately scaled elevation data. A design team has the greatest ability to influence the overall cost of a project during the early phases of the project.

Inaccurate (too coarse) data can easily be obtained over a larger area, but can lead to the poor decision making, potentially resulting in costly revisions to preliminary alignments. Highly detailed data can be acquired but at much greater expense. Such costs can only be mitigated through tightening the project area, introducing the risk of moving an alignment “off the map” and creating further expense through the requirement for subsequent data capture missions.

A DEM derived from satellite imagery offers a competitive compromise between accuracy and extent, for feasibility and preliminary design work. Once an alignment starts to become more stable, the capture of higher resolution data can be undertaken in a more strategic manner, at a much reduced cost than a speculative approach over a larger area.

Elevation data can also be used to great effect in a route selection process to more accurately predict costs associated with number of crossings to avoid areas that present unmanageable construction safety hazards (slope >15-20º) or to assist in locating suitable sites for key infrastructure (pump stations, balance tanks) in water pipelines.

Elevation data can also be highly useful in geomorphologic investigations along a corridor. The identification of ephemeral watercourses or indicators regarding unmapped geological structural elements can inform a design team of possible erosion hazards, assist in stratifying field investigations to identify stable geological units to construct upon or identify potential extraction material sources.

Proposed Access Road (2008 Landsat) Proposed Access Road (WV-2) © DigitalGlobe 2010

Hillshade derived from WV-2 DSM with known watercourses. DSM generated from WV-2 with known watercourses.

Ephemeral watercourses identified from DSM and hillshade are verified in VHR imagery and added as environmental constraint on road selection criteria.

Hillshade layer allows geomorphic interpretation of area. Ephemeral watercourses visible in image that are not mapped, but present erosion risks to road development

Proposed alignment amended to minimise regional ecosystems protected under the Vegetation Management Act (2009)

Regional Ecosystem mapping refined using GEOBIA of WV-2 imagery allows further adjustment to alignment and minimising the time needed for field survey

Prediction of Bushfire HazardThe threat of bushfire is a very real hazard across most populated parts of Australia. As such the predictive assessment of bushfire risk is critically important information for land developers, Local and State Governments as well as in the planning of major infrastructure.

In this case study, a rural residential area was selected that contained a variety of landcover types (Woody Vegetation, Cleared/Grassland, and Residential), contained a mixture of lot sizes and a noticeable range of relief. 8-band multispectral imagery was captured by WV-2 in stereo mode, which facilitated both the automated generation of a DSM and Geographic Object-Based Image Analysis (GeOBIA).

The scale of the derived DSM should be determined by the intended uses for the data and desired scale of these applications. In this example a 5m DSM was selected to dampen the variability (noise) from VHR imagery.

Three key products were generated in this example: 1) Elevation and derivative Slope and Aspect surfaces; 2) Landcover mapping based on combustibility; and 3) a Fire Hazard Layer derived from the analysis of these derived surfaces. The Fire Hazard was further summarised by cadastral parcel, allowing reporting of risks or hazards at either a Local Government or property scale.

The visualisation of these layers and analysis outputs in 3D (based on DSM height) gives the user further power to interrogate the outputs and increase their understanding of the resultant information.

WV-2 Imagery - Natural Colour

Combustability of Landcover generated with GEOBIA

DSM generated from Stereo-pair

Slope and Aspect derived from DSM

Combined Fire Hazard Surface

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Geoimage has been offering professional and independent advice on the supply, processing, analysis and integration of satellite imagery and spatial datasets since 1988. Geoimage’s unsurpassed reputation for timely delivery, dedicated follow-up and professional customer service is evident from our extensive client list spanning the mining and exploration, environmental, planning, engineering and government sectors.

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• Advanced spectral processing for feature extraction, including Object Based classification and analysis

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typical applications that benefit from the establishment of a geospatial framework include:

• Natural Resource Management including habitat value and environmental risk assessment

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