An Accuracy Assessment of a Digital Elevation Model Derived From an Airborne Profiling Laser

1. An Accuracy Assessment of a Digital Elevation Model Derived From an Airborne Profiling Laser Joseph M. Piwowar Philip J. Howarth Waterloo Laboratory for Earth Observations University of Waterloo Waterloo, Ontario, Canada Elaine Lindo Multimedia Consutling, Inc. Toronto, Ontario, Canada

2. Rationale • many applications (e.g. precision farming or SAR backscatter calibration) require the use of highly accurate (i.e. errors<10 cm) digital elevation models (DEMs) from which local slope and aspect measures can be derived • existing techniques for deriving DEMs (e.g. photogrammetry or ground surveys) are labour intensive and may be prohibitively expensive in the commercial marketplace • DEMs calculated from airborne laser profilometers are potentially cost-effective solutions for surveying large areas but their accuracies have not be extensively evaluated

3. Objective • to evaluate the accuracy of a DEM created from data acquired by an airborne laser profiling system with one generated with a differential GPS ground survey Study Area • data were collected over a series of agricultural fields in southwestern Ontario, Canada, during the spring of 1998 • the results for one of these fields are presented here; findings from the other fields were very similar • the test field covers an area of 31.4 ha and has a relative relief of 4 m

4. Reference "dGPS" Data • differential GPS (dGPS) data were collected by mounting an Ashtech Z–12 GPS receiver on a roving ATV which was systematically driven up and down each field • position fixes were acquired at 2 sec. intervals • individual passes down the field were spaced at about 9 m • a second Ashtech Z-12 GPS receiver was used as for the base station • theoretical position accuracies of the GPS setup were 3 cm horizontal and 5 cm vertical

5. Survey Grids dGPS ALTM

6. Test "ALTM" Data • airborne laser profilometer data were acquired by an Optech Airborne Laser Terrain Mapper (ALTM) system mounted in a fixed-wing aircraft • survey swaths from three aircraft passes were required to cover the field • data from the ALTM have an elevation accuracy of better than 15 cm with spatial positioning at about 10 cm, using differential GPS

7. Analysis Method • dGPS and ALTM data were received as XYZ text files with X & Y coordinates referenced to a UTM projection at 1 m resolution, and elevation (Z) values specified with 1 cm resolution • empty, georeferenced DEMs were created large enough to span each field at 1 m resolution • the dGPS and ALTM elevation data were used to seed the DEMs with known values • remaining empty DEM grid locations were filled in by linear interpolation (PCI task GRDINT using the DIAGONAL search algorithm) • a difference image was calculated by subtracting the ALTM data from the dGPS data

8. Results • the test field ranges in elevation from about 215 m to 219 m and covers an area of 31.4 ha • the in-field elevation variability (i.e. elevation standard deviations) of the two data sets are closely matched • the survey point densities vary dramatically from 1 point / 101 m2 for the dGPS data to 1 point / 13 m2 for the ALTM data

9. DEMs dGPS ALTM

10. Analysis • there is a slight, but abrupt, change in relative image elevations following a line running from northwest to southeast down the centre of the field • southwest of the line, dGPS data are higher; above the line, ALTM data are prominent • noted at point "A" on Transect 1 • ALTM data reveal a more undulating terrain northeast of the line • source of this inconsistency is unclear • may be related to a slight change in direction of the survey lines collected by the GPS instrument • the actual elevation shift is very subtle and does not affect the quality of either DEM

11. there are 2 circular anomalies at locations "B" and "C" • Transect 2 shows that at point "B" ALTM values are about 1 m higher while at point "C" dGPS values are greater by about 1 m. • similar anomalies do not appear elsewhere in the data set • the source of these anomalies is unknown • improper flight management left a 50 m wide gap between data swaths crossing the north section of this field • elevation "streaks" are visible where the gridding software attempted to bridge across the data gap • this does not have a significant impact on the integrity of the ALTM DEM • the standard deviation of the differences in elevation between the 2 data sets is about 30 cm • ~68% of all the grid points differ by less than 30 cm

12. A B Difference Image C

13. Conclusions • both data sources provide accurate results • ALTM detects micro-scale roughness • ALTM seems to be more tolerant of data errors; a 5 m wide gap between adjacent swaths across a field did not noticeably affect the derived DEM. There is more human intervention in the collection of the dGPS data which raises the possibility of additional errors. • There do not appear to be any consistent patterns of departure between the DEMs derived from either data source that, if present, would have indicated a bias inherent in one of the procedures. Thus either data source can be used. • These results are corroborated by similar analyses on four additional fields, not reported on here.