This paper presents a matching algorithm for automatic DTM generation from SPOT images that provides dense, accurate and reliable results and attacks the problem of radiometric differences between the images. The proposed algorithm is based on a modified v

Figure 5.

Top row: left (left) and right (middle) SPOT image at 4th pyramid level and normalized dif-ference image (right). Bottom row: binarized difference image (left), image with selectedpoints (middle), image with cleaned selected points (right)

5. DERIVATION OF APPROXIMATIONS

AfterthePMFsarecomputedanaverageheightisusedinordertodeterminethepositionoftheselectedpointsinthesearchimage.Tocheckthequalityoftheseapproximationsthe136pointswereprojectedontothesearchimagebyusinganaverageheightof1000m,andthesepixelcoordinateswerecomparedtotheknownones.TheRMSdifferenceswere32pixelsinxand2pixelsiny,withthemaximumerrorbeing72and5pixelsrespectively.Thus,are nementoftheseapproximationsbyanim-agepyramidapproachisnecessary.AnalternativeapproachwouldbetoactuallytransformandresamplethesearchimagebyusingthePMFsandtheaverageheight.Inthiscase,thedisadvantagesare(i)thecomputationalcostsforthetransformationandtheresampling,and(ii)thedegradationofthedata.Theadvantagesinclude:(i)matchingcanbeperformedusingonlyshifts,thusresultingincomputationalgainswhichincaseofmanypointsexceedtheloses,(ii)detectionofradiometricdifferencescanbeappliedasproposedabove,and(iii)sincethey-parallaxoftheco-registeredimagesisverysmall,theimagescanbeviewedstereoscopically (which is anyway required in digital photogrammetric workstations).

6. ACCURACY TESTS

Theaccuracyofthematchingalgorithmwastestedbyusingthe25mDTMofSwitzerlandwhichisgeneratedbytheFederalOf ceofTopography.TheDTMsofthe1:25000mapsheets1224and1225wereacquired.EachDTMhas701x481nodesinE-WandN-Sdirectionrespectively.TheDTMsareproducedbybicubicinterpolationinxandydirection,wherebytheknownheightsaresuppliedbydigitisedcontours,lakecontoursandspotheights.TheaccuracyoftheseDTMswascheckedbybicubicinterpolationofca.1000spotheightsandcomparisontotheirknownvalueswhichhaveanaccuracyofca.1m.The1224DTMwasderivedfromca.107000heightvaluesandhasanaccuracy(RMS)of1.9m.Theheightrangeis900mbuttheterrainis