Re: GIS Update

[mskala at ansuz dot sooke dot bc dot ca]

On Tue, 21 Sep 2004, D. Cooper Stevenson wrote:
>   1) We are not after satellite photos, we are after "Digital Elevation
> Models."

Well, depending on what you want to do with it.

A DEM is a height field.  It specifies the elevation at a bunch of points.  
If you are building a map that cares about elevation, then you want that
for sure, but you might want other kinds of data too.  For my Antarctic
map I also used the DEM to infer the surface type, because that seemed the
most useful thing to do, but that was something of a kludge adapted
specifically for the Antarctic map.  On other maps there are probably
other kinds of data that can provide better guidance as to the final "bin"
to put the hex in.

For instance, if we had multispectral data (like from a LANDSAT TM image)
we could use that to classify hexes by type of surface terrain - forest,
cropland, water, etc.  That could be useful.  LANDSAT TM is like an image
file with five or six channels of colour information (corresponding to
colour bands from visible into infrared, with some subtleties and
complications) at a resolution where each pixel is 15 to 25 metres.  I
don't know that we could get that for free, but I'm pretty sure we can get
AVHRR data (similar kind of thing, pixels are kilometres in size) for
free.  If we had "line coverages" of vector data for things like rivers,
it might be fun to try to convert those into border or connection terrain,
although the complexity of the algorithms to do that might get pretty
high.  Part of the fun is that there are lots of different kinds of GIS
data out there.

>   2) Because the data is in a raster format (as apposed to a regular
> image file),  I believe it is quite possible to automate the creation of
> new maps and smaller scale views of those maps

One thing to watch out for is that not all DEMs are really rasters.  Some
are just lists of points; others are complicated data structures called
Triangulated Irregular Networks or TINs, and even if they are rasters,
they may not be on the grid or projection you want.  GRASS has some pretty
sophisticated tools for manipulating the data into whatever grid you want,
so that shouldn't be a problem, but it'll be something to keep in mind.
-- 
Matthew Skala
mskala@ansuz.sooke.bc.ca                    Embrace and defend.
http://ansuz.sooke.bc.ca/