Creation and Evaluation of Digital Elevation Models from Ground Surveying Measurements

Fahmy F. F. Asal
Page No. 15-29

Creation of a Digital Elevation Model (DEM) from point support files requires exploitation of an
interpolation approach in order to get a continuous surface forming a DEM. The Inverse Distance
Weighting (IDW) and the thin plate spline methods are two local interpolation methods that are very often
exploited in the creation of DEMs. Different interpolation techniques have to be expected to provide
different quality DEMs. This research has been focused towards evaluation of the quality of the DEMs
generated from ground surveying measurements using IDW and thin plate spline. A test site in a hilly
corrugated terrain area has been established and digital elevation measurements have been collected from
field using conventional ground surveying methods where a total station instrument has been used for
measuring the three dimensional coordinates (x, y, z) of the selected spot points. DEMs have been created
from the field data using commercial spatial analysis systems; in addition qualitative analysis of the DEMs
has been undertaken aiming at viewing differences between IDW DEM and the spline DEM in representing
the earth’s surface. Furthermore, quantitative analysis has been carried out using sets of external checkout
points uniformly distributed over the test area. Visual analysis has shown that the IDW DEM possesses wide
variations in the tone/colour throughout the DEM with rough texture compared to the thin plate spline DEM
which shows narrower variations of the tone/colour and finer texture. The statistical analysis of the DEMs
has shown that IDW DEM has given a minimum elevation, a maximum elevation and a range of elevations
of values that have been very close to those of the original test data set while the corresponding values from
the spline DEM have been very different from those values of the original data. The 3D view from IDW
DEM has shown corrugated surface while that of the spline DEM has also shown a corrugated surface but
with clear elevation spikes that can be interpreted as elevation interpolation noise. The accuracy assessment
using external checkout points has shown that the maximum residual from IDW DEM has been greater than
that from the spline DEM while the absolute minimum residual from IDW DEM has been smaller than that
from spline DEM. Additionally, the absolute algebraic sum of residuals from IDW DEM has been much
smaller than that from the spline DEM. Finally, the standard error of the extracted elevations and the
standard error of the mean of the extracted elevations from IDW DEM have been smaller than their
corresponding values from the spline DEM which refers to that IDW algorithm has generated more accurate
digital elevation model compared to the DEM that has been generated by the thin plate spline approach.
Key words:- DEM/DTM/DSM, Ground Surveying, IDW, Thin plate spline, Digital Mapping,
Spatial Analysis.