If it were me, I would import both the GPS measurements and a scan of my
original map into a GIS system and compare. Maybe the stations are not
where you thought they were, maybe you have a projection or reference
system error, or maybe the original map was a little off. So, you might
also take your GPS unit to a couple of control points and then see how
their location plots on your original map.
On 7/14/2011 10:43 PM, Carol Boggs wrote:
In the pre-gps era, I calculated the area of one of my study sites using a high
resolution USGS map. Recently we checked that value using a GPS. For the part
of the study site that is fairly flat, the numbers match up very well.
However, for the part that has a significant downhill slope, they do not. The
difference is striking - the gps-derived value is nearly 40% smaller than the
USGS map-derived value. The gps values were differentially corrected and done
with a Trimble that should be accurate enough that I would not expect a
difference of over a hectare between the map and gps area values.
Does anyone have any experience or advice for dealing with area calculations in
areas of ~3-5 ha, involving significantly uneven ground?
thanks,
Carol
Carol Boggs
Dept. of Biology
Stanford University
summer:
Rocky Mtn. Bio. Lab
Crested Butte, CO
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