1 INTRODUCTION

Many of the data which are used in GIS come fromexisting graphical products printed onto paper, orfrom images derived from photographic cameras ordigital sensors (e.g. see Bibby and Shepherd, Chapter68, Waters, Chapter 59). Some of the latter comesfrom sensors carried on satellites. In order to usethese data in a GIS they must be processed to be in aform which is compatible with the GIS. The firstpoint to consider is whether the GIS requires rasteror vector data – assuming that the GIS cannotconvert between the two without imposingundesirable constraints. The second point concernsthe reference system: all data which are to beoverlaid, merged, or compared must be in the samemap projection and on the same datum. This impliesthat any distortions present because of the datacapture system must be corrected. The third point isthat the quality of the data must be known (seeVeregin, Chapter 12). This chapter is concerned with

how vector or raster data can be acquired fromexisting maps or from images and how they must beprocessed in order to be used in a GIS. In order tocarry out these processes it is essential that thecharacteristics of the original data and the processesused in the preparation are understood. It is alsoessential that all of the possible sources of error arerecognised, particularly with regard to how sucherrors affect the quality of the final dataset (see alsoFisher, Chapter 13; Heuvelink, Chapter 14).

The term ‘map’ is used in this chapter to refer to adataset which contains accurate information aboutfeatures on the surface of the Earth. In the pastthere would have been no ambiguity about thisbecause a map user would be handling a piece ofpaper with the required information printed onto it.Now this information may also be stored in digitalform (see Longley et al, Chapter 72, for a discussionof the ‘map metaphor’).

A knowledge of map projections is essential whenhandling data from different sources. Any map

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31Encoding and validating data from maps and images

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This chapter gives an overview of how data for a GIS can be derived from existing graphicalinformation (a map) or from image data such as aerial photographs and satellite images. Twocrucial principles underlie this process: (a) the need to understand the characteristics of theoriginal data, including the quality of the source data; the processes used to produce thedocument to be encoded; and the geometric characteristics of the data; (b) the need tounderstand the processes used to produce the encoded data and their subsequent quality.

In order to obtain this understanding, this chapter gives some detail of thecharacteristics of the data sources and of the methods of converting the original data into aform suitable for a GIS. The need to geometrically correct image data leads to the use ofelevation data to correct for relief effects and therefore the principles of using stereoscopicdata are outlined and methods for deriving digital elevation models. It is assumed thatautomation will be used in these processes as much as possible, so an outline is given ofthe automated tools which are now in use. Finally, the question of accuracy is discussed,including the factors which affect accuracy, how accuracy can be assessed, and how thisinformation can be given to and applied by the user of a GIS.

projection will introduce some distortion into thetransformation from the ground to paper and it isimportant that the sources of this are understood.Data mapped using different projections cannotgenerally be combined and processes of conversionand transformation must therefore be applied. Inaddition different datums may be used, even on thesame projection, and these too must be understood.A detailed discussion is beyond the scope of thischapter but reference may be made to Snyder (1986;see also Barnsley, Chapter 32; Seeger, Chapter 30).

The first step in using map data in a GIS is toencode it into vector or raster format. This involveseither line scanning or raster scanning. Because ofthe monotony of this task, it is desirable that asmuch automation as possible is introduced. Afterencoding, the data must be transformed into therequired projection and any known errors removed.A raw image derived from a sensor in an aircraftor on a satellite will contain distortions attributableto the attitude of the sensor and the relief of theground, and it will not show all the features whicha user will need (Barnsley, Chapter 32; Meyers,Chapter 57). Such distortions must be correctedbefore data derived from the images can be used in adefined reference system. Correction may be possiblethrough 2-dimensional transformation, although3-dimensional correction using stereoscopic imageryor digital elevation models (DEMs) is frequentlyalso necessary. Such processes are part of thediscipline of photogrammetry and this section canbe regarded as an introduction to that subject.

A vast amount of useful data already exist in theform of printed maps and an immediate response tothe requirement for digital data is to assume thatthese existing data should form the basis of thedigital database. It must, however, be rememberedthat a great deal of effort, and hence cost, is requiredto convert printed maps to digital data. A figure of60–80 per cent is often given, withoutsubstantiation, as being the proportion of the costof establishing a GIS which is attributable toobtaining the data. It must also be remembered thatmaps may be out of date and may have a number oferrors inherent in them, and that the conversionprocess will itself add further errors. Beforeembarking on a data acquisition exercise it issensible to weigh up all the factors involved in datacollection, and to consider the available sources ofdata in the context of the use to which the data will

be put and the methods which will be used to updatethe data. Logan (1995) provides a useful discussionon these topics.

2 ENCODING MAP DATA

2.1 Principles

Map data have two principle components: geometryand attributes. For use in a GIS every point in adataset must have unique coordinates (x and y or x, y,and z) and each must be labelled. On a printed mapthe coordinates usually come from a grid or graticuleon the map, provided in such a way that the (x, y)coordinates of any point can be read off a scale. Theinformation about what the point represents will comefrom the context or the way in which the point isrepresented, and this representation can be convertedto an attribute through a legend. When convertingfrom a printed map to a digital database, both of thesecomponents must be recorded.

Digitising will always take place in a rectangularcoordinate system which is defined by the digitisingsystem. Conversion from this to a national orregional grid system is therefore relativelystraightforward. The procedure is usually to recordthe corner points of the document to be recorded inthe digitising system and then to carry out atransformation (see section 5.3) between therecorded coordinates and the known coordinates inthe national system of the corner points. An affinetransformation in often used for this to correct forany errors created by paper distortion. Digitising isoften carried out in patches (or tiles) which may bethe size of a grid square or larger and each tile istransformed to the coordinates of its corners. It isalways wise to carry out checks with well-definedpoints of detail on the map.

Attributes must be assigned by the operator, orautomatically where possible, through the use of anattribute menu or key coding.

2.2 Equipment

The most straightforward form of conversion is todigitise manually the information from the printedmap into either point, line, or area features and toassign each feature an attribute code. The featuresare then recorded as coordinates in point or vectorform. This can be done on the digitising table on

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which the map is placed. The operator then selectspoints or trace lines and the coordinates arerecorded as the cursor moves over the map. Afterdigitising, the result needs to be carefully checkedand edited to ensure that everything has beenrecorded and that lines join up where they shouldand do not overrun. If carried out systematicallyand carefully this method can result in accurate andcomplete vector data. The disadvantage is the timewhich is required and the tedious nature of the task.

A map can be quickly and easily recordedautomatically in digital form using scanningtechniques. However these methods only record thedata in terms of position in a raster format and donot link to separate attribute files, since there is nofeature coding or topology. Scanning can beaccurately carried out on drum scanners, whereby theprinted document is fixed to an accurately rotatingdrum. The drum is scanned by an optical readinghead which determines whether a line or symbolexists over a ‘pixel’ (picture element) of a given size –which can be as small as 6µm. Colour documents canbe recorded by scanning three times, in the red, green,and blue regions of the spectrum. The result is araster image which reconstructs the original as pixelsrather than as lines and symbols and which may lookthe same as the original but cannot be interpreted bya computer. The accuracy and visual impression iscontrolled by the pixel size, but a smaller pixel sizemeans more data, more time to scan and display, andincreased storage requirements. Current problemswith scanning are centred on accurate colourreproduction and the size of files which will beproduced for large documents. The former problemcan be overcome at the expense of greater datahandling if colour separates are available.

Less expensive but less accurate desktop scanners,designed for the publishing trade, are available whichwill scan at up to 1200 dots per inch (giving a pixelsize of 20.16µm). Desktop scanners are subject togeometric distortion but this can mostly be removedby calibrating the scanner and subsequent correction(Sarjakoski 1992).

2.3 Raster-to-vector conversion

The raster image can be converted to vector by araster-to-vector conversion. This technique is notfully developed as an automatic process and still

cannot add attributes. It also requires extensiveediting to remove text and symbols. Raster-to-vectorconversion requires salient pixels to be assigned(x, y) coordinates. The actual coordination of a pixelis no problem but deciding which pixel shouldrepresent a corner or a junction of two lines is noteasy. A junction, for example, may be represented byseveral pixels and the lines forming the intersectionmay be several pixels wide; the intersection itselfmust, however, be represented by a single point. Linethinning will reduce line size but an algorithm isneeded to select the optimum point to represent theintersection. This is illustrated in Figure 1, where thenode of the intersection could fall at any of the fourpixels within the small box. Features such as roadsmay be represented as single lines or as parallel lines,and the decision as to which to use and how todetermine which lines to digitise can also be difficultfor an automatic system.

The most efficient solution for raster-to-vectorconversion at present may be semi-automaticsystems. These are based on line-followingtechniques which can be done quite well by acomputer. The operator is better at starting theprocess and making decisions about which way to goat intersections of lines. The VTRAK system ofLaser-Scan Limited (Cambridge, UK) is a goodexample of an automatic line-following system: it

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Fig 1. An example of a problem in raster-to-vector conversion.

can be used to digitise features into a range ofdatabases, notably object-oriented databases. It alsoprovides editing and quality control facilities andautomatic text recognition.

2.4 Editing and error analysis

Editing is an important process in any digitisingoperation because errors will always be made. Themost common error is poor closing of lines atintersections or in polygons. Most digitisingpackages now include efficient tools for editing data(see Bernhardsen, Chapter 41). Figure 2 shows someexamples of common errors.

Errors in content have been discussed above andcan be detected by careful checking. Errors ingeometry are not so easy to detect. The mostimportant rule in checking accuracy is to haveotherwise redundant check points. As explained abovea transformation is necessary between the digitisingsystem and the reference system. Usually four pointsare a minimum requirement to carry out atransformation. A number of additional check pointsshould also be used and the coordinates oftransformed points in the digital document should becompared with the original values. Some usefulvalidation statistics are discussed in section 8. Afurther useful check is possible if other data of thesame area exist and the two sets can be superimposed.

3 THE GEOMETRY OF AERIAL PHOTOGRAPHSAND SATELLITE DATA

3.1 Image formation

In order to produce a map from any satellite image itis necessary to first define the geometry of the imageforming system, to consider the movement of theplatform on which the sensor is mounted, and todefine the shape of the ground which is covered bythe image. The importance of each of these factorswill depend upon the type of sensor used and thepath which the sensor is following relative to theground. This section is concerned with the generaleffects and these are related to different sensor types.Specific sensors for mapping are discussed in section4 and by Barnsley (Chapter 32) and Estes andLoveland (Chapter 48).

Images from a particular sensor are usuallypresented in a standard format covering a definedarea of ground. Such an image is called a frame. Aframe may be formed in three ways:

1 As a single exposure – that is with no significantmovement of the sensor whilst the image isformed as is found in the case of a frame camera.This is used for central projection which is used inall aerial photography and in a number ofsatellite sensors.

2 As a series of lines almost normal to the track ofthe sensor. In this case time must be considered inthe model for defining the construction of a fullframe. The main types of sensor in this categoryare the push broom scanners, of which SPOT isthe best contemporary example.

3 As a series of points each recorded at a separatetime. This is the most distorted type of imagerequiring the most complex mathematical model.The scanner systems such as LANDSAT fall intothis category.

The photographic camera, in which an image isformed by light rays from an object being focusedonto a focal plane by a lens, may be taken as thestandard case. This lens acts as a point throughwhich all light rays pass, so that the process of imageformation can be seen as a series of straight linespassing through the perspective centre S onto thefocal plane. If the objects all fall on a plane which isparallel to the focal plane, then a direct reduction ofthe object is found in the focal plane and the amountof reduction is given by the ratio of the object

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Fig 2. Common errors in digitising.

Gap in closingpolygon

Overlap inclosing polygon

Overshoot

Missing segmentError in coordinate

causing a spike

distance to the image distance. This is anapproximation to the case of vertical aerialphotography when the ground is the object, theobject distance is the flying height H, and the imagedistance is the principal distance of the camera f(equal to the focal length of the camera when theobject distance is effectively infinity). Two points aredefined on the vertical line passing through S as thenadir points on the image (n) and on the ground (N).The scale of the photograph in this case is f/H. Thisis illustrated in Figure 3. In practice the ground isnot flat and the focal plane is not horizontal.

3.2 Effect of relief

The effect of relief is to cause the scale to changethroughout the image and to cause images to bedisplaced, radially from the nadir, from the positionat which they should appear if there were no relief.In Figure 4 a point A on the surface of the groundshould appear on a map of scale f/H at a' butactually appears on the photograph at a.

The magnitude of this displacement is given bythe expression

aa' = an . h (1)H

or in terms of a radial distance from the nadir points(r)

dr = r . h (2)H

This expression indicates that the effect of reliefdisplacement is less from greater sensor altitudes.For example on an aerial photograph taken from10 000 metres, relief of 500 metres will cause adisplacement of 5 mm for a point appearing on theedge of a photograph (r = 100 mm). This isequivalent to 660 metres on the ground if a wideangle camera is used. On an image taken from asatellite pointing vertically downwards, with analtitude of 700 km the effect of relief on the groundwill be 64 metres. For normal relief the effect on thistype of satellite image can be ignored, but forphotographic cameras and tilted satellite sensors itcannot be ignored and 3-dimensional geometry mustbe considered. In Plate 19 the effect of reliefdistortion can be clearly seen at the edge of thephotograph where buildings appear to leanoutwards. In other words the top of the building isin a different position to the bottom.

The effect of relief can be removed by using arigorous 3-dimensional model of the geometryof two images or by having a digital elevationmodel available in order to compute, and hencecorrect, the effect of relief at every point. An imagewhich has been corrected for the effect of relief iscalled an orthoimage.

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Fig 3. The scale of an image.

n

N

S

f

H

Fig 4. Distortion attributable to relief.

a a' n

A

h

H

3.3 Distortion because of tilt and other causes

There are other distortions in imagery which mustalso be considered. These include the tilt of thesensor platform, the movement of the platform andthe movement of the Earth. Most of these are small,although they still need to be removed if accuratedata are required. A source of distortion which ispotentially very serious arises if the sensor is tiltedthrough a large angle – for example SPOT imagescan be tilted by up to 27 degrees and the new highresolution sensors will tilt by up to 45 degrees. Thisis an effect which must be removed for accuratemapping. The methods of correcting distortion aredescribed in section 5.

3.4 Acquisition of stereoscopic data

If images from two different positions areavailable, then the pair of images can be viewedstereoscopically to give a 3-dimensional view of theterrain and measurements can be made to give3-dimensional coordinates. In order to obtain astereoscopic image from a pair of photographscertain conditions must be satisfied. The camerasmust neither be too close together, nor too far apart.If they are too close together the view will not bedifferent enough to enable a stereoscopic image to beformed; if they are too far apart the views will notbe similar enough. Photographs should also betaken with the same or similar cameras and be takenfrom approximately the same distance from theobject. The basic geometric condition can beexpressed in terms of base to height ratio.

In Figure 5 the base is shown to be the distancebetween the cameras, B, and the distance betweenthe base and the object is the height. In aerialphotography the distance is equivalent to the flyingheight, H, and we have a base to height ratio, B:H.In order to obtain good stereoscopic viewing B:Hshould lie between the limits 0.3 and 1.0.

Photographs taken with a camera from anaircraft for the purpose of constructing a map arecarefully controlled so that the axis of the camera ispointing almost vertically downwards and eachphotograph overlaps its neighbour by 60 per cent.This ensures complete coverage of the ground and a

convenient base to height ratio. This is shown inFigure 6. Stereoscopic coverage from satellitesensors is more variable and because of the greataltitude of the sensor coverage from verticalpointing sensors does not usually give a suitable baseto height ratio for mapping.

3.5 Heights from stereoscopic data

As already noted, stereoscopic images can be usedfor measuring heights. The principle is shown inFigure 7. On two images, two points of differentheights, A and B, will produce images at a1, b1 on theleft hand photograph and at a2, b2 on the rightphotograph. The separation of the images a1p1 +a2p2 and b1p1 + b2p2 is clearly proportional to theheights of A and B. These separations are calledparallaxes or disparities.

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Fig 5. Base to height ratio.

B

H

Fig 6. Base to height ratios of: (a) aerial photography;(b) vertical satellite images.

H

B

(a) (b)

Using the principle as shown in Figure 7 we can derivean expression which relates the height of a point (Z)to the camera geometry and the parallax (p)

Z = f B (3)p

and considering a small change in height (dZ) wecan show that:

dZ =Z . Z dp (4)f B

Equation 4 is an expression which shows how asmall change in parallax (dp) will give a smallchange in height (dZ). This equation can be usedto give the precision with which height differencescan be found. Precision is usually measured by thestandard deviation of a set of measurements, orquantities derived from measurement. In this casethe standard deviation of height determination, szcan be found if the precision of observation ofparallax sp is known and the equation becomes:

sz =Z . Z sp (5)f B

which gives an expression showing that theprecision with which height can be measured isrelated to the scale of the photograph (Z/f), thebase-to-height ratio (Z/B), and the precision ofmeasurement of parallax.

4 IMAGE ACQUISITION SYSTEMS

Images for mapping may be obtained from sensorsplaced in aircraft or on satellite platforms. Airbornesensors are mainly film cameras producing highquality images suitable for photogrammetricmapping. Sensors from satellites generally produceimages directly in digital form as the satellite movesforwards: the movement enables the necessary largevolumes of data to be recorded, but complicates theimage geometry. A summary of the main sensors isgiven in Table 1. The characteristics of satellitesensors are discussed in more detail by Barnsley(Chapter 32) and Estes and Loveland (Chapter 48).Here only a few of the sensors which can be used formapping will be discussed. Film cameras are used inspace and the Russians are the main operators ofsuch cameras. The KFA1000 and KVR1000 are twoexamples which are included in Table 1. Both ofthese are designed to give large scale images ratherthan height information.

Scanners are the most widely used imaging systemsfrom space, and LANDSAT is probably the bestknown. A scanner records a single pixel (or a group ofpixels) at a time; a strip approximately at right angles tothe direction of flight is imaged as a mirror rotatesabout the axis of flight. The raw data obtained from ascanner are subject to many distortions which must becorrected prior to use within a GIS, but because manyof the sensors point only directly downwards, rigorouscorrection is not required.

Pushbroom sensors are the most important formapping. SPOT was the first of these and wasdesigned as a mapping sensor: it is still dominant inthis area, although other systems have been designedand flown. The US high resolution sensors are animportant new form of pushbroom scanner.

Stereoscopic overlap can be arranged either foreand aft or laterally so that 3-dimensional modelscan be formed. Displacement attributable to therelief of the ground is normal to the direction offlight, that is along the arrays, and the amount ofdisplacement is proportional to the distance fromthe principal point and the altitude. If lateraloverlap is used by tilting the sensor than the effectof relief will be large, but a good base to heightratio will be obtained for height measurement.

The SPOT satellite uses a ‘lateral overlap’ sensor.This gives a good base-to-height ratio but suffersfrom the disadvantage that stereoscopic images will

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Fig 7. The principle of height determination.

B

a1 b1 p1 p2 b2 a2

A

be taken a minimum of one day apart – usuallylonger, as cloud conditions may change from oneday to the next. ‘Along track’ stereo imaging is moreefficient as it is very likely to generate stereoscopicdata. The optical sensors (OPS) on JERS-1 andMOMS-02 are along track systems: other systemshave been designed and will be implemented in thefuture. Of particular note are the high resolutionsensors such as Earlybird and a system from SpaceImaging, being developed by commercialorganisations in the USA. An example of simulateddata is shown in Plate 18, which is part of SpaceImaging’s CARTERRA San Francisco series of highresolution, high accuracy imagery. It was derived byfusing a one metre resolution panchromatic imageand a four metre resolution multispectral image.Laid on top of a United States Geological Survey(USGS) seven-and-a-half minute quad sheet, thisimage illustrates the value of high resolutionimagery in place of traditional line-drawn maps.They are used for a variety of applications, includinginfrastructure management, urban planning,utilities, and transportation.

Another type of sensor is synthetic aperture radar(SAR). Images from these sensors have a quite

different geometry and appearance to those of othersensors and details can be found in specialistliterature on the subject (Schreier 1993). Radar isbecoming particularly important at the momentbecause of its potential to produce high accuracyelevation data through SAR interferometry.

5 CORRECTION OF ERRORS IN IMAGES FORUSE IN GIS

5.1 Requirements

The quality of image data to be used in a GIS mustmatch that of the use of the data. Although manyapplications require only 2-dimensional data,3-dimensional information may nevertheless beneeded to apply the necessary corrections. Thereare two main methods of correction. First, a2-dimensional transformation which is suitable forapplication to images in which there is very low reliefdistortion – as, for example, in LANDSAT data.Second, a rigorous 3-dimensional correction whichcorrects for all the effects of relief, sensororientation, Earth rotation, and Earth curvature.

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Table 1 Characteristics of sensors used for mapping (see also Barnsley, Chapter 32 Table 2).

Platform Sensor Launch Type Pixel Swath B:H Heightdate size width accuracy

(m) (km) (m)

Aircraft Film camera Principal manufacturers † 0.3 – Depends of are Zeiss and Leica 1.0 flying height:

0.01% overhead

Cosmos KFA1000 1994 Russian film camera, 5–10* † 0.12 30f = 1000mm

Cosmos KVR1000 1987 Russian panoramic film 2* 160 0 N/Acamera, f=1000mm

Landsat Thematic Mapper 1982 Scanning system 29 180 N/A N/A

SPOT 1-4 HRV 1986–97 Cross track push broom 10,20 60 ≤1 10

JERS-1 OPS 1992 2 sensor along track stereo 20 75 0.3 50

Priroda MOMS 1996 3 sensor along track stereo 6 40 0.9 3–6

IRS-1C Pan 1995 Cross track stereo 5.8 70 1 5–10§

Earlybird 1997 Along track stereo 3 6 1.2 1.5‡

Quickbird 1997 Along track stereo 1 36 1.2 1.5‡

Space Imaging 1997 Along track stereo 1 11 2 0.4‡

Orbview 1998 Along track and cross 1–2 8 2 0.4‡track stereo

† Varies with altitude * Originally photographic film but available in digital format.§ Preliminary result ‡ Predicted result

For both of these methods ground control points(GCPs) are generally required.

5.2 Ground control

A GCP must be recognisable in both images (or oneimage if stereo is not being used) and have knowncoordinates in a suitable ground reference system.The number of points required depends on themethod used (Seeger, Chapter 30). GCP coordinatesmay be obtained directly by survey measurement orfrom a map. In either case the coordinates will begiven in a reference system; this may be geographical(latitude and longitude) or Cartesian (x, y, z) and itmay be global, based on the centre of the Earth, orlocal, based on a national or regional projection.It is always important that the characteristics of thereference system are known and that all coordinatesare given in the same system. Further information ofreference systems and conversion between systemsmay be found in Seeger (Chapter 30).

Direct survey measurements may come fromsurveys based on a local or national coordinatesystem (e.g. the UK’s National Grid), or they maycome from the Global Positioning System (GPS)which allows coordinates to be fixed directly fromnavigation satellites (Lange and Gilbert, Chapter33). Maps should be used with caution fordetermining GCPs. Map data at scales of 1:25 000and smaller are notoriously unreliable because ofthe many errors which may have accumulated in themap production and map digitising process. Theseinclude survey errors (in some parts of the worldpublished maps may be based on topographicalsketches), drafting errors, generalisation, paperdistortion, and errors in digitising a paper documentfor use in the validation process. While it is alwaysnecessary to take the accuracy of the data intoaccount when using ground control, it is particularlyimportant when using map data.

5.3 Plane transformations

5.3.1 IntroductionFor areas of low relief or for low resolution sensors,fairly simple methods of correction may be used.The correction of data in two dimensions may beapproached by applying a transformation to the dataand resampling the data to produce a corrected

image which gives a best fit to the ground controlused. The transformation may be based on atheoretical consideration of the errors involved ormay be selected on empirical grounds. The method isthe one most commonly used to produce an imagewhich is corrected to fit to a given map projection.

A number of transformations are widely used anda brief description of the common ones is givenhere. An image coordinate system (x, y) and aground coordinate system (X, Y) is assumed.

5.3.2 Two-dimensional similarity transformation(four parameters)

This transformation is to relate any 2-dimensionalrectangular coordinate system to any other2-dimensional rectangular coordinate system. Itpreserves the internal geometry of the transformedsystem, so it is ideal for comparing the geometry ofany two systems simply by determining the residualsand the root mean square errors after transformation.For a given control point, the two equations:

X = ax – by + c (6)

Y = bx + ay + d

may define this transformation.A similarity transformation is performed by

applying a scale factor (m = (a2 + b2)), a rotationangle (tan α = b/a) and two translations (c and d).A minimum of two GCPs is needed although itis always desirable to have more for purposesof checking.

5.3.3 Two-dimensional affine transformation(six parameters)

A mathematical relationship for an affinetransformation may be expressed in the following:

X = ao + a1x + a2y (7)

Y = bo + b1x + b2y

An affine transformation enables adjustment to beapplied independently in each direction. Thus forscanner images it corrects first-order distortions suchas affinity attributable to non-orthogonality andscale difference between scan along track directionswhich may be caused by earth rotation, mapprojection, and other geometric distortions.A minimum of three GCPs is required.

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5.3.4 Second-order polynomials (twelve-parameters)Polynomials in the form:

X = ao + a1x + a2y + a3x2 + a4y

2 + a5xy (8)

Y = bo + b1x + b2y + b3x2 + b4y

2 + b5xy

are used for correction of scanner data. Ifpolynomials are used, great care must be taken toensure that a sufficient number of control points isavailable and that they are distributed over the wholearea to be transformed. A minimum of six GCPs isnecessarily required to determine the transformationparameters, although it is desirable to have more tobuild in checks. In addition to first-order distortions,polynomials correct second-order distortions causedby pitch and roll, subsatellite track curvature andscan lines convergence because of Earth rotationand map projection. They may also correct some ofthe distortions related to the attitude variationsalong the flight path.

Additional terms may be added to Equation 8 tocorrect for higher order distortions. The need for carein use of control points is greater for higher orders.

5.3.5 Resampling After calculating the parameters of thetransformations, the transformation must be appliedto the image and new Gray level values be computedfor each pixel in the operation of resampling.Resampling may introduce some changes to the dataand distort the characteristics of some features.

5.4 Determination of orientation elements

The process of extracting 3-dimensional coordinatesmust include the determination of the orientationand position of the sensor and the application ofthis information to the measured image coordinates.The information required comprises the position ofthe sensor given in (X, Y, Z) coordinates in anappropriate reference system, and the attitude of thesensor given as rotations about the referencecoordinate system. These six parameters are knownas the elements of exterior orientation. They can befound in a number of different ways:

1 By measurement on the platform or sensor. Theposition can be accurately determined with somesatellites, for example ERS-1 where the orbit isaccurately determined, or if a GPS receiver isused. There is no suitable system for determiningattitude accurately.

2 With the use of GCP. The position and attitudeof the sensor can be calculated by relating theimage to ground control points. This is relativelystraightforward for photographic images, but ismore complex when the image is formed over aperiod of time as with push broom sensors. Thesensor geometry and orientation is describedmathematically and the unknown parametersdetermined by a mathematical solution such as abundle adjustment.

5.5 Principle of correcting height distortion

If the relief is sufficient to cause distortion (section3.2), then a correction must be applied. The processof distortion due to relief shown in Figure 4 can bereversed to correct for the effect. Figure 8 shows thesame geometry as Figure 4 but the terrain is depictedby a series of points, P1, P2, . . . . Pn on a regular grid.Each point has a planimetric position (X, Y) and aheight (Z), and together they form a DEM. If acorrected image (orthoimage) is to be formed then thecorrect plan position of the point Pi , that is Pi', willfall at pi". To find the correct Gray level value theposition pi in the image is found by projecting Pi intothe image. The Gray level value at pi is placed at pi" byresampling. In practice this is done as a mathematical3-dimensional transformation taking into account theposition and attitude of the sensor. The process cantake place in a digital photogrammetric workstationor using an image processing package. These arediscussed in section 2. The topic of digital elevationmodels is discussed in section 3.

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Fig 8. The correction of relief distortion.

Orthoimage atknown scale

pi pi' n

f

pi"

p1 p2 p3 p4pi pn

W pi'

6 THE PRODUCTION OF SPATIAL DATA

6.1 Methods

The basic geometry of 3-dimensional imagingsystems has been described in sections 3 to 5.This may be incorporated into a computer system inorder to allow the extraction of features or height bymanual or automatic means. Traditionally, opticalmechanical instrumentation has been used byphotogrammetrists for map making but this has givenway in recent years to analytical instruments in whichthe operator views the stereoscopic model and followslines or measures heights under computer control.A degree of automation has also been introduced.Completely digital systems are now being developedin which stereoscopic data acquisition can be carriedout as part of a digital image processing system. Suchsystems allow for stereoscopic viewing andautomation is also integral to them.

To use an aerial photograph in a digital system,the photograph first has to be scanned. This is doneusing a scanner similar to those described insection 2.2, but designed especially for digitisingaerial photographs with geometric precision of1 – 2 µm and pixel size of down to 7.5 µm.

Digital images can also be viewed with astereoscope on a split screen display or with theanaglyph principle in two colours. Better qualitydigital display can be obtained by using polaroid or‘flicker’ display on the monitor. In these systemsglasses must be worn. For a polaroid display thespectacles have complementary polarisation and inthe ‘flicker’ display the glasses are synchronised withthe monitor. Stereo viewing systems are available asan option on workstations.

6.2 Digital workstations

Digital photogrammetric workstations (DPWs) havedeveloped from analogue and analytical plottinginstruments designed to produce maps from aerialphotographs, and from image processingworkstations designed for handling satellite data forinterpretation and analysis. They are designed tohandle stereoscopic data in digital form and willinclude software which carries out a number offunctions automatically. There will normally be afacility for stereoscopic viewing and for automaticmatching of images to produce heights. The mainfunctions are:

● Orientation of stereoscopic images● Feature extraction and assignment of attributes● Generation of DEMs● Creation of orthoimages.

Some digital systems have been developed primarilyfor photogrammetric work, but others have beendeveloped from GIS or image processing systemsdesigned for handling satellite data. Some, such asIntergraph and ERDAS, are closely linked to GISand data can be easily transferred between systems.There is also a difference in cost and functionalityand accuracy of the products from such systems.Table 2 includes the features of some of thesesystems. A full discussion of digital workstationswith references to detailed descriptions can be foundin Dowman et al (1992).

One of the major innovations of digitalworkstations has been the introduction of automaticprocessing of data, including the formation ofdigital elevation models by automatic stereocorrelation (image matching). The basic principle

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Table 2 Some digital photogrammetric systems.

System Comment

Leica HAI-750 Two full photogrammetric systems on Unix or PC platforms. Can use SPOTLeica HAI-500

Zeiss PHODIS Full photogrammetric system for use with aerial photographs and satellite data.

Intergraph ImageStation Photogrammetric data capture as part of digital mapping system. Can use SPOT.

ERDAS Imagine with Orthomax Designed for production of orthoimages with DEM generation. Linked to ARC/INFO.Can use aerial photographs or SPOT.

R-Wel DMS PC based system for DEM and feature extraction from images as input to and integration with a GIS.

Leica DVP PC based system for digital mapping from aerial photographics and SPOT.

}

underpinning stereo correlation is the matching ofsmall patches from two images: the amount ofrelative displacement to achieve matching isdetermined and this is used to determine parallaxdifference which in turn is used to compute heights.The distortion of the patches because of relief andtilt must be taken into account. Errors will occurbecause of features such as trees and buildings noton the terrain surface, and matching may beincorrect or impossible if the two images aredifferent because of changes on the ground or largediscontinuities in the terrain surface.

This can be a particular problem with SPOT whendays, weeks, or months may pass between imagesbeing obtained. Accuracies of better than 10 metrescan be obtained from stereo matching SPOT data.Line maps have been produced from SPOT usingtraditional photogrammetric methods. OrdnanceSurvey in the UK and Institut GéographiqueNational (IGN) in Paris have produced maps at1:100 000 and 1:50 000 scale using analyticalplotting instruments (Smith and Rhind, Chapter 47).Image maps are, however, the more usual product,and can be overlaid with vector data.

Plate 19 shows a stereoscopic pair of aerialphotographs in which the buildings appear to leanoutwards – thereby demonstrating the effect ofrelief displacement. This effect is most marked onthe left image, which appears in the corner of thephotograph from which it was taken. The fiducialmarks on the top of the images are used to orientthe photographs, and they can be viewed stereoscopically using a pocket stereoscope. Part ofPlate 19 shows a plot of part of the area producedon a photogrammetric plotter and an example ofthe output file for two of its features – a lamppost(marked A), which is stored as a symbol along withits coordinates, and a node on a line (marked B)which is part of a (hatched) building and isattributed a line type (solid) and a colour.

6.3 Digital elevation models

6.3.1 Introduction DEMs are a way of presenting the elevation of thesurface of the Earth in numerical form. A DEM willconsist of a series of reference heights arranged inregular or irregular form. A typical regular pattern isa rectangular grid. The spacing of the grid will,together with the accuracy with which the heightsare given, be a function of the overall accuracy of

the DEM. Digital elevation models can be derivedfrom a number of sources and the processes whichare required may differ according to the source. Amethod of derivation will normally be related to theapplication of the DEM. The most accurate DEMmay give heights to an accuracy of a few centimetreswith a high density of reference points, but coveringa small area. At the other end of the scale, globalDEMs may have a spacing of several kilometres andgive heights to an accuracy of hundreds of metres.This section will deal with methods of derivingDEMs from aerial and satellite data. Petrie andKennie (1990) give a full account of DEMs.

6.3.2 ProcessesThere are three main processes associated with DEMs:

● primary data acquisition;● resampling to required grid spacing;● interpolation to extract height of required point.

Each of these stages may introduce errors, the natureof which will depend upon the type of terrain andupon the method used. The data may be acquiredfrom satellites in the form of heights fromstereoscopic measurements, from interferometricmeasurement or from direct measurement with aranging device. In the case of stereoscopicmeasurement, automatic correlation will normally beused. In general these methods will produce heights ina quasi-systematic pattern, rows or patches on anapproximate grid pattern. These must be resampled toa regular grid. Plate 20 shows a colour coded view ofa DEM and a perspective view. The image to the left isa colour coded DEM of the area between Marseillesand Grenoble in southeast France, and is derived fromSPOT panchromatic images: the heights were derivedon a 50-m grid and are accurate to about 10 metres inheight. The image on the right is a shaded view of thesame area, derived from the DEM.

DEMs may be generated from stereoscopic aerialphotographs or satellite images. The accuracyattainable will depend on the scale of the photographor on the pixel size. Manually derived DEMs fromaerial photography can provide very accurate andreliable information, but the process of recording it istime consuming and tedious. Automatic methods arenow widely used but suffer from the problem that alarge amount of checking and editing may still berequired. The extraction of heights can be doneautomatically in an off-line mode. Complete scenesof SPOT data can be stereo matched in a few hours

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in order to obtain a DEM which can be used toproduce orthoimages and image maps.

In recent years SAR interferometry has been usedto create DEMs. This is still an emerging technologybut very good results have been reported in someareas. The method has the advantage of beingindependent of cloud and also that differentialmethods can detect shifts in terrain, because ofearthquake for example, to a few millimetres.

7 ACCURACY OF DATA

Many aspects of data accuracy have been coveredindirectly already, and are also discussed by Fisher(Chapter 13) and Heuvelink (Chapter 14). Whenencoding map data, the principle source of error is themap itself. Encoding systems can reproduce thefeatures on the map with high apparent precision, butthe error present in the original document will remain.These errors are attributable to a number of factors:

● original survey;● revision;● generalisation;● drafting and reproduction.

All available information about the source of themap and the data used to compile and revise itshould be collected where possible and stored alongwith the encoded data. Thus, although no reliablemeasure of the accuracy is available, at least asensible estimate of the error can be made.

The errors in images will be dependent on the scaleof the image, or the pixel size, and on how rigorouslythe corrections have been applied. A fully rigorousphotogrammetric plot will remove all distortions andthe errors will be directly related to the scale of theoriginal imagery. An orthoimage produced with agood quality DEM will also have small errors.However, images covering hilly terrain, corrected witha polynomial, may have large errors present. Errors inthis type of product will also be subject to the qualityof the ground control which may have been poorlyidentified, or the coordinates might be in error. Whenregistering two datasets, both sets may contribute tothe accuracy of the registration.

8 VALIDATION OF ENCODING PROCESSES

Validation can be carried out by evaluatingconsistency, precision, and accuracy. Accuracy is

defined as the closeness of a measurement to the‘truth’. In practice ‘truth’ usually means the bestavailable data. Accuracy can be evaluated by theroot mean square error which is given as:

rmse = √ Σv2

n (9)

where v is the residual error, the difference betweencomputed and reference ground coordinates, and n isthe number of observations. Precision is a measureof the closeness of measurements to each other andis defined by the standard deviation of a set ofmeasurements:

σ = √ Σ(v–v- )2 (10)n–1

where v is the mean of the residuals. For furtherdiscussion of RMSE mesures, see Beard andButtenfield (Chapter 15) and Fisher (Chapter 13).

Consistency is a more subjective measure,depending upon human interpretation of whatlooks correct.

9 FUTURE PROSPECTS

The growing use of GIS and the increasing volumesof data from new sensors make it increasinglyimportant that data are corrected and referenced toa common system. These corrections need to be asaccurate as possible and need in particular to correctfor topographical effects. This increases the need forappropriate DEMs. In the future, systems for imageprocessing and image analysis will almost certainlyinclude software to produce orthoimages andDEMs: this latter requirement means that new datasources to produce DEMs are necessary.

A number of missions are planned which aredesigned for producing stereoscopic data formapping. The ideal parameters for a mapping satelliteare discussed by Light (1990). The US high resolutionsatellites are the most exciting of these developmentssince they will produce very high resolutionstereoscopic data to produce high precision (x, y, z)coordinates, orthoimages, and DEMs.

Another development of increasing importance isthe automation of processes. Automatic DEMgeneration has been discussed and is a productionprocess. Automatic registration of images is alsopossible and work on image-to-map registration is inprogress. Extraction of some coverage features suchas roads is a problem with considerable potentialrewards, but is still at the research stage.

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References

Dowman I J, Ebner H, Heipke C 1992 Overview of Europeandevelopments in digital photogrammetric workstations.Photogrammetric Engineering and Remote Sensing 58: 51–6

Light D L 1990 Characteristics of remote sensors for mappingand earth science applications. Photogrammetric Engineeringand Remote Sensing 56: 1613–23

Logan I T 1995 Cost and benefit considerations in datacollection and the application of data collection techniques.Paper presented at Cambridge Survey Officers’ Conference

Petrie G, Kennie T J M 1990 Terrain modelling in surveying andcivil engineering. Caithness, Whittles Publishing

Sarjakoski T 1992 Suitability of the Sharp JX-600 desktopscanner for the digitisation of aerial colour photographs.International Archives of Photogrammetry and RemoteSensing 29(B2): 79–86

Schreier G (ed.) 1993 SAR Geocoding: data and systems.Karlsruhe, Wichmann

Snyder J 1986 Map projections – a working manual. ProfessionalPaper 1395. Washington DC, US Geological Survey

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