Geoscience Reference
In-Depth Information
the surface of the rock glacier provided ideal image contrast. Displacement vec-
tors were then divided by the period covered between image sequences to estimate
flow (m/yr).
To visualize change, the mid-points of the flow vectors were then converted to
a continuous field using ordinary kriging. Because velocities of rock glaciers tend
to decrease from the head to the toe, a global trend was removed from the dataset
to improve kriging estimates in a west to east direction, following the main axis of
the rock glacier. The same interpolation criteria were applied to each image set to
avoid misinterpretation from different interpolation techniques. The interpolations
were then subtracted using map algebra to visualize horizontal change in the rates
of flow between 1983 to 1988 and 1988 to 1998.
Thickness change (vertical displacements) was measured by subtracting the 1983
and 1998 DEMs in ArcGIS. The 1988 DEM was not utilized in this portion of the
analysis because uncertainty was too high given a short period of time (5-10 years)
between the dates of the images. Uncertainty was measured by averaging horizontal
(
x
and
y
) and vertical (
z
) error between stable points surrounding the rock glacier.
The average error terms were then divided by the number of years in the record to
express uncertainty results in m/yr.
5.3.2 Field GPS Measurements
GPS measurements were taken with a submeter GPS receiver for seven rocks on
the surface of California rock glacier. Points were marked with neon paint and pho-
tographed in 2003 to aid in relocation. One stable GCP was marked off the surface
of the rock glacier to measure the error associated with the recordings. During 2008,
the points were relocated and another set of GPS measurements were taken. Points
were then post-processed and differentially corrected. Rates of flow (m/yr) were
calculated in ArcGIS by measuring the distance between points and dividing by the
number of years in the record.
5.4 Results
5.4.1 Photogrammetric GIS Measurements
Photogrammetric measurements indicate that the rates of horizontal flow averaged
57 cm/yr (
3 cm/yr) from 1983 to 1998 (over the entire period). Velocities gen-
erally decrease from the head toward the toe of the rock glacier. An average rate
of horizontal flow of 66 cm/yr (
±
8 cm/yr) from 1983 until 1988 was calculated
(Table
5.2
,Fig.
5.3
). From 1988 to 1998, average horizontal velocities decreased to
55 cm/yr (
±
4 cm/yr) for the entire rock glacier (Table
5.2
,Fig.
5.4
). Between the
two periods, velocities showed the most substantial decrease near the midsection of
the rock glacier (Fig.
5.5
).
±
