Field Visits
I visited the study area three times in 2003. On April 4th and 5th I took photographs and recorded elevations. On April 25th I rechecked all elevations and took additional photographs. I had to recheck elevations because I found that my measured elevations were inconsistent. My altimeter, built into the Silva GPS unit, works with atmospheric pressure. Thus the altimeter measures changes in air pressure resulting from two sources: changes in altitude and changes in the weather. Unless the effects of these two variables can be separated the altimeter will measure their combined effect, hence the altitude reading will not be the true elevation.
My solution is to locate mapped benchmarks and to return to these benchmarks periodically during the visit. The real altitude does not change but both barometric pressure and the nominal altitude readings change hourly. With the recorded figures for the benchmark to a precision of one meter I can adjust the nominal altimeter readings of nearby sites to get their true elevations with a precision of one meter. In practice, the calculated real elevations may be accurate to no more than plus or minus two meters, but this was deemed to be within acceptable limits.
Using the software package IDRISI
I added grid lines to the Series
L8010 topographic map of the study area. Using
these grid lines, spaced 30 seconds apart, together
with the Silva
GPS unit, I was able to locate features on the
ground in relation to elevations shown in the DEM.
Since the study area is near the Equator, the cells
of the 30-second grid are approximately one kilometer
square, thus one-tenth of distance between grid lines
equals 100 meters. The only topographic map available uses latitude and longitude. I therefore
set the GPS unit to display degrees, minutes, and
hundreds of minutes. One minute is about two kilometers
and 0.01 minute equals about 20 meters. The effect
of this approach allowed me to estimate the location of a point on
the map to about 100 meters and then to locate myself
on the ground within 20 meters of that point using
the GPS unit. Some objects could be located more precisely, such as main
road intersections and small temples.
The map shows the locations of field sites with clickable areas defined by boxes linked to descriptions and photographs.
