Module 13 - Special Topics in GIS - Effects of Scale

We began the last 3-week topic in Special Topics for this semester, on issues of scale and resolution in GIS.  As an introduction, we examined how resolution decreases in data as the scale at which it was collected  decreases:  in other words, 1:100,000 scale data will have much less detail than the same features recorded at 1:1,200 scale.

We examined two different types of DEMs in this exercise: one obtained at 1-m resolution by LIDAR and the other obtained at about 82-m resolution by SRTM, which is space-shuttle-borne microwave, known also as RADAR.  I resampled the LIDAR 1-meter data to pixel sizes of 2, 5, 10, 30, and 90 meters, and compared their slopes.  The highest resolution data (1-m) will have the highest-value slope and the slope will decrease slightly with decreasing resolution and increasing pixel size.  This is because with more and smaller pixels, the slope for each across a given horizontal distance will be higher, resulting in a higher average slope, than the slope across a single larger pixel.
The LIDAR 90-meter DEM was then compared to the SRTM DEM, which was reprojected to the same resolution (90 m) and coordinate system. The elevation and its derivatives (1st-order: slope and aspect, and 2nd-order: curvature) for both 90-m DEM's were compared to those values for the original 1-meter LIDAR DEM, with the assumption that the higher resolution original data is more accurate.

The relative errors for all values (elevation, slope, aspect and curvature) between the 90-m LIDAR and the 1-m LIDAR are smaller than those between the SRTM 90-m DEM and the 1-m LIDAR.

Selected Results from comparison to LIDAR 90-m DEM and SRTM 90-m DEM, to LIDAR 1-m DEM

It can be noted from the values in the above table that while average differences in elevation between the LIDAR 90-meter and SRTM 90-meter are quite small, the differences are compounded in the 1st derivative of slope, which hinges on elevation.  The differences are even greater for the 2nd derivative of curvature.

These results make sense, because LIDAR has many more elevation data points because of the much more rapid data pulse rate.  It is also a much shorter wavelength than SRTM.