[GRASS-user] slope in lat long

[Gerald Nelson]

The exchange below reminds me to ask a question that is not really about GRASS. Both GRASS and the main competitor want x,y, and z units to be the same when computing slope, or want the user to input a conversion between the xy and z units. I would have thought that if the software knows that x and y are in lat long and that z is in meters it could do the slope calculation. But there seems to be no way to tell either GIS that z is in meters and automatically do the slope. So I project to some utm system and do it there. Is there a geographic reason for why the software packages don't allow this or am I not finding the right set of commands in them?

Thanks, Jerry

---- Original message ----
>Date: Thu, 25 Jan 2007 20:22:54 +1300
>From: Hamish <hamish_nospam at yahoo.com>  
>Subject: Re: [GRASS-user] barscale with lat/long projection  
>To: "Brandon M. Gabler" <bgabler at email.arizona.edu>
>Cc: grassuser at grass.itc.it
>
>Brandon M. Gabler wrote:
>> I'm using GRASS 6.2, and want to add scale/north arrow to maps. My  
>> location is in lat/long projection, and I'd prefer to leave the  
>> location that way - is there a way to do this? Or will I need to  
>> start a new location with a different projection, and import my files 
>> to that?
>
>
>As scale is different in lat/lon NS and EW directions (Plate Carrée
>projection) drawing a distance scale is not allowed. As you move north
>or south, the horizontal scale changes as cos(lat). (other popular GIS
>software may let you draw one, probably due to high customer demand, but
>it is still very misleading to do so)
>
>Use d.grid instead. Or d.graph with a compass symbol if you like:
>G63> echo "symbol extra/fancy_compass 75 5 90 black red" | d.graph
>maybe with a "N" above that (d.text).
>
>I have just made a change in 6.3-cvs so you can do "d.barscale -n" from
>a lat/lon location without error. [devs: I think it would be better to
>rewrite the -n flag to use a symbol and then display that with
>D_symbol() vs. current way using direct screen drawing commands.]
>
>Another code change might be to modify d.barscale to report distance
>units in degrees for lat/lon, not meters/feet.
>
>
>
>Hamish
>
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Gerald Nelson
Professor, Dept. of Agricultural and Consumer Economics
University of Illinois, Urbana-Champaign
office: 217-333-6465 
cell: 217-390-7888
315 Mumford Hall
1301 W. Gregory
Urbana, IL 61801