Geoscience Reference
In-Depth Information
Fig. 5.2
Terrestrial photograph of the California rock glacier
5.3 Methods
5.3.1 Photogrammetric GIS Measurements
A combination of GIS, remote sensing, and field techniques were used to measure
horizontal and vertical displacements from 1983 to 2008. USGS digital aerial pho-
tographs with a resolution of 0.3 m were obtained for 1983, 1988, and 1998. Leica
Photogrammetry Suite (LPS) was used to create DEMs and orthophotos for each
image date. Full Ground Control Points (GCP) (x, y, and z) were located in the
field (at least 5 per image) and were then digitized on the digital stereopairs. Tie
points (non-field points, but points easily identifiable on each image) were gener-
ated to improve stereocorrelation estimates. A DEM was then created for a region
around California rock glacier using a special filtering strategy for high mountain
terrain.
To create orthophotos for each image, the DEMs were used to remove the effects
of topography, and a cubic convolution technique was used to resample the image.
The corrected images and DEMs were imported into ArcGIS to assess horizontal
and vertical change. A cross-correlation technique was used to identify correspond-
ing points on each image. A 27 by 27 cell (8.1 m by 8.1 m) filter was passed over
the set of images. Once a correlation of greater than 90% existed on the set of three
images, the centers of the filter were identified (0.3 m) and vectors were drawn
to connect the centers of each filter. This method is highly dependent upon suffi-
cient image contrast so that corresponding pixels can be identified. Large rocks on
