>One way of solving you problem is to assign each base class with a 'Magic'
>number. You could use factors of 2. e.g. 1,2,4,8,16,32,...,1024,2048...
etc.
>The neat thing with these numbers is that if if you add them they act as a
>logic OR. And from the result you van always deduce which were the
origional
Thanks Roeland,
Your suggestion has merit, but is not that different to the other
mathematical solutions that I have tried. The problem with this is that
vertical mapper sems to fray the boundaries of classes during an overlay
when the grid is represented as a numerical grid. When doing the overlay of
two grids that may not be completely coincident, it seems that as the system
queries a grid for a pixel that is slightly within an area of 2048 but
mainly in an area of 2, it may return a result of 32 or some other number
that does not mean anything specific. This would naturally invalidate any
assumptions of what classes mean. It would be OK if I could add class grids
where a grid can have one of a discreet set of values.
I take back what I said before, numerical calculations would probably not
work even if there were rounding and integer functions in the grid
calculator tool.
Now to confound the whole saga, when I tried to extract the areas of the
grid defined by each possible combination of the 2 layers to give a set of
subgrids, I get an error -
Vertical mapper Error #1802
Extended error value 2
Have no idea what this means, but am now going back to creating a grid of
points from the slope grid and write the vegetation data as attributes to
that based in a simple update query. I can then use standard databse queries
to give me my answers. I have been using this technique for close on 8 years
now (started on the DOS MapInfo), and its slow, but it works.
R
PS at least VM is very fast in exporting data - it exported a 3 million
point coverage in the time it took me to compose this letter. I suspect that
mapInfo will take the rest of the night to update the vegetation class to
that grid.
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]]On Behalf Of Roeland van der Spek
Sent: Monday, April 03, 2000 7:59 PM
To: Robert Crossley; Mapinfolist
Subject: MI Re: Vertical Mapper Polygon overlay
One way of solving you problem is to assign each base class with a 'Magic'
number. You could use factors of 2. e.g. 1,2,4,8,16,32,...,1024,2048... etc.
The neat thing with these numbers is that if if you add them they act as a
logic OR. And from the result you van always deduce which were the origional
base class numbers. Exactly like the bits of binary number.
An obvious limitation however is the number of classes that you want to
assign. 32 bit numbers are standard so 32 classes are no problem...
Roeland
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