I am trying to intersect a slope category map with a vegetation fire risk
map to produce a third coverage showing unique combinations of slope risk
and vegetation hazard. The resultant risk is not a simple mathematical
function of the two, and we wish to assign classes and descriptions of risk
manually for each combination.
The tables contain very complex polygons and the approach of splitting one
coverage by the other, or using a program which I have written that does
this in steps will not finish in this lifetime.
I have been trying to use vertical mapper to do much of the analysis to
date, but am now stuck.
Vertical mapper does not allow grid calculations using character based
fields, so I cannot simply create new classes based on the two character
based grids.
If I try to use numbers instead of characters to designate classes and do
calculations to give unique results eg. use prime numbers for the base
classes and multiply the grids, or multiply one coverage by 10 and add the
second; I get interpolated numbers for each layer along the edge of each
class area that no longer directly point to any one of the previous classes
(thus I cannot guess what combination of base layers that this area came
from). This approach would probably work if there were rounding functions
or integer functions available in the grid calculator.
I am now selecting each combination and selecting out areas that have each
unique combination of each of the two grids, eg. make areas where vege class
of 1 and slope class of 1 = 1 and other areas be null. I will have to do
this 25 times (once for each combination of classes, and then splice the
partial results together. This is a pain to do, and its success will be
dependant on how null values are treated when splicing grids.
Any suggestions?
R
Robert Crossley
Trinity Software
10 Trinity Street
CAIRNS 4870 AUSTRALIA
Ph: 61-7-40314877
[EMAIL PROTECTED]
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