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1 August 2009 Professional Surveyor Magazine | www.profsurv.com

M

ulti-cone rame cameras extend the range o

photogrammetric applications; through digital

rame cameras, photogrammetry has been rein-

vented and will continue to evolve. Meanwhile,

we have not even scratched the surace o what

digital photogrammetry can do. Future opportunities in en-

hanced processing sotware will extend digital-camera sensor

capabilities to more and more applications. There is huge po-

tential in what can be called sotware leveraged hardware.

One visible and important application is DSM production

rom digital imagery. All major sotware vendors are imple-

menting algorithms to automatically extract surace models

rom digital imagery. Vexcel Imaging, or example, has devel-

oped ully automated ortho-rectication sotware and auto-

mated 3D model generation algorithms or Microsots Bing

Maps (ormerly Virtual Earth). These sotware developmentswill process thousands o UltraCam images or generating 3D

city models or Bing Maps.

aerial perspective|by Alexander Wiechert and Dr. Michael Gruber

Photogammty sus Lda: Clang th A

Figure 1: An xampl of mult-ay photogammty

Figure 2: A ss of hgh-olappng 10cm GSD mags sult n a dns sufac modl (>50 ponts p squa mt) though automatd dnsmatchng.

Multi-ray PhotogrammetryMulti-ray photogrammetry has created a signicant change

in photogrammetry with the advent o the digital camera and a

ully digital work fow. This allowed or signicantly increased

orward overlap o images as well as the ability to collect moreimages virtually and without increasing acquisition costs, be-

cause only hard disk storage and computation time is required

to store and process the additional imagery, and both are low-

cost. Its a signicant improvement compared to lm-based

cameras where each image aects direct costs such as lm, de-

velopment, and scanning.

Multi-ray photogrammetry is not exactly a new technology,

rather a specic fight pattern with a very high orward overlap

(80 percent, even 90 percent) and an increased sidelap (up to

60 percent). The result is considerable redundancy, critical or

robust automated matching. One pixel on the ground is visi-

ble in up to 15 images. Such a dataset allows automated dense

matching to extract surace models rom the imagery.

Once the DSM has been processed rom the imagery, a l-

tering and classication processcomparable to the ltering

and classication o elevation models acquired by lidar scan-

ningcan be applied to achieve the terrain model (DTM).Figure 3 shows a greyscaled relie o the DSM (let) and the

DTM (right). The DSM has been processed using UltraMap

sotware and a set o UltraCam images. The DTM has then

been processed out o the DSM using a Winston-Salem algo-

rithm developed by Microsot.

High-resolution DSM and DTM production is no longer

a domain o lidar scanning and can be accomplished also

through photogrammetry and digital rame cameras. In most

applications, multi-ray photogrammetry achieves a signi-

cantly higher point density with superior collection eciency.

The achievable height accuracy is better than the GSD; thus a

10cm imagery leads to


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2www.profsurv.com | Professional Surveyor Magazine August 2009

Figure 3: H a hgh-soluton DSM (lft) and DTM (ght) pocssd usng automatd dnsmatchng wth UltaCam mags and UltaMap pocssng softwa.

Figure 4: 24cm UltaCamL mag

Figure 5: Hgh-soluton DSM

DSM pocssd out of th mags

10cm GSD tu-otho mag

Figure 6: High-resolution DSM 10cm GSD tu-otho mag

In Figure 4 the let image shows

a part o the city o Gleisdor near

Graz, captured at a ground sample

distance o 24 centimeters. The city

has been surveyed with an 80 per-

cent orward overlap and a 60 per-

cent sidelap. This allowed automatic

processing sotware to acquire a sur-ace model rom the imagery with an

average point density o more than 8

points per square meter. The DSM is

shown on the right.

Another survey was perormed

with 10cm imagery (Figure 5). The

raw imagery was then processed and

a very dense DSM automatically cre-

ated employing the multi-ray pho-

togrammetry approach and through

dense matching o the imagery. The

average point density o the DSM is

higher than 50 points per square me-ter and the height accuracy is better

than 10cm. The DSM was then used

to process a true-ortho image rom

the raw imagery.

In Figure 6 the image on the let

shows a grey shaded relie o the

dense DSM that was processed by

automatically matching the digital

imagery rom the UltraCamL fight.

The grey values represent height in-

ormation. The right image shows the

true-ortho image that was then pro-

cessed by overlaying the raw imagery

with the dense DSM.

Noticeable is the extremely sharp

edge representation in the DSM due

to the high point density achieved

through the automated matching o

the high-resolution imagery. The re-

sult is that the true-ortho imagery

shows almost no artiacts in spite o

the 10cm high-resolution GSD.


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3 August 2009 Professional Surveyor Magazine | www.profsurv.com

Project ComparisonThe table below compares a lidar project with an image proj-

ect. The goal o the scenario project is the creation o a DSM

with a density o 8 points per square meter. This translates into

a 25cm GSD image collection project or the cameras. A gener-

ic high-perormance lidar sensor system has been chosen or

the comparison with the UltraCamXp and UltraCamLp.

A e r i A L P e r S P e C T i v e

The comparison shows that with a high-perormance li-

dar sensor system, an eective strip width o 322 meter can

be achieved, compared to 1,731 meters when using the Ultra-

CamXp and 1,170 meter through the UltraCamLp. Setting the

UltraCamXp strip width to 100 percent, the collection ecien-

cy o the UltraCamLp is 68 percent o that and the lidar sensor

system is 17 percent.

FPO

(1/4Ad)

The multi-ray photogrammetry approach extends the ca-

pability o photogrammetry or applications currently being

served through lidar scanning. In combination with multi cone

rame-based digital cameras, it allows an automated point-

cloud and DSM generation rom digital images through ro-

bust dense matching. Due to the superior collection capacityo digital rame cameras, the rame-image based DSM gener-

ation leads to higher point densities at lower collection costs

when compared to lidar scanning. A DTM can be processed by

applying ltering and classication comparable to the ltering

o lidar data or the DSM.

Whats NextA uture improvement o the camera-based DTM produc-

tion will be achieved by combining traditional ltering and

classication with image-based classication. Image-based

classication allows automatic extraction o more inormation

about objects. This inormation can be used to improve the l-

tering and the classication o the DTM data.

An additional and signicant benet is that users will be

able to process DSM and DTM rom digital rame images with-

out an additional sensor or an additional workfow. Theyll be

able to do so with pre-collected digital rame images and the

standard photogrammetric workfow already in place.

AlexAnder Wiechert is general manager at Vexcel ImagingGmbH, a Microsoft company in Austria. He holds degrees in Aero-space and Aeronautics and in Business Administration.

dr. MichAel Gruber is chief scientist at Vexcel Imaging GmbH.

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