continuous thermal data and r.slope.aspect

[Alastair Duncan]

Hello,

I have a raster image showing sea surface temperature as a continuous surface
derived from a digital thermal sensor. The image is calibrated so that one unit
represents 1/16th of a degree Celsius. There is quite a range in temperature
from ambient to ambient plus 7 degrees Celsius, where discharges from industria
l
sources and rivers flow into the sea - these are marked by both sudden and very
noticable rises in temperatures that form water body boundaries, and by mixing
zones where temperature changes gradually.

Using a slope map generated from r.slope.aspect, treating the thermal image as
the elevation model, you can very clearly see where the mixing zones and water
body edges are - however I'm trying to figure out what the units generated by
r.slope.aspect actually mean in terms of temperature change per metre. If I set
up r.slope.aspect with the following parameters :-

r.slope.aspect elevation=thermal_image slope=slope_map format=degrees prec=floa
t
zfactor=1

I get a slope_map with range from 0 to 79.0797, however with format set to
percent I get a range from 0 to 518.3016.

Does anybody know what these values would mean in terms of temperature change
per metre ? The resolution is 8 metres, and setting the zfactor to 1 means that
1/16th of a degree Celsius is being treated as 1 metre in the r.slope.aspect
program.


Alternatively, am I barking up the wrong tree, if so does anybody know a way of
calculating rate of change in terms of unit of parameter per unit of distance
for a continuous surface other than actual elevation ?? 

Cheers, Al.

Alastair Duncan,
GIS Data Officer,
The National Centre for Environmental Monitoring and Surveillance,
The Environment Agency,
Bath, UK