Date:         Fri, 7 Mar 2003 15:00:29 -0600
Reply-To:     Pete Earls <[log in to unmask]>
Sender:       Biological Conservation and GIS <[log in to unmask]>
From:         Pete Earls <[log in to unmask]>
Subject:      Re: Projections and GTOPO30 data
In-Reply-To:  <[log in to unmask]>
Content-Type: TEXT/PLAIN; charset=US-ASCII

Rob,

I have not heard that DEMs should not be reprojected.  Do you know what
the problem with doing this?  Each cell value is the value at just the
center of the cell, so reprojecting corrupts the DEM definition due to
resampling.  On the other hand, the single-point elevation values in the
original DEM seem unrepresentative for their respective cells to begin
with, especially for course-resolution DEMs (GTOPOs e.g.) where the
terrain could vary quite a bit in a single cell.

The GTOPOs are in decimal degrees, which is not suitable for area
analysis, so shouldn't the GTOPOs be reprojected for this reason?  Also, a
map in decimal degrees for a large part of the world is very unappealing.

- Pete

==================================================================
Peter Earls
Oklahoma State University
[log in to unmask]


On Thu, 6 Mar 2003, Robert A Norheim wrote:

> Dorn --
>   I've just started playing with GTOPO30 in ArcMap myself, and am having
> the same problems.
>   It sounds as if you are interested mostly in making maps, rather than in
> doing analysis.  My recommendation would be that you reproject the DEMs
> into projections that are more appropriate for your specific maps.  I
> haven't tried it so I don't know that it works, but we might be asking too
> much of ArcMap to do the projections on the fly.  So perhaps doing the
> projections ahead of time might work.  You could also resample the grid
> spacing when you reproject to something that will match the scale of the
> map better so you don't have huge reprojected grids laying around.
>   Normally, one should avoid reprojecting DEMs when doing any analysis
> with them.  However, if you're just doing cartography, I don't think it
> will be so much of a problem, especially if you resample to a coarser grid
> spacing.
>
> --Rob
>
> Robert A. Norheim
> GIS Analyst
> University of Washington
> Seattle, WA  98195-2100
> [log in to unmask]
> http://staff.washington.edu/norheim/
>
> Multiple affiliations and phone numbers at UW:
>
> Fire and Mountain Ecology Lab, 206 543-9138
> (College of Forest Resources)
>
> Fire and Environmental Applications Team, 206 932-7809
> (USDA Forest Service, Pacific Northwest Research Station)
>
> Pacific Northwest Climate Impacts Group, 206 616-6865
> (Joint Institute for the Study of the Atmosphere and Ocean)
>
> "Time spent climbing mountains will not be deducted from your life"
>
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