slope calculation, was RE: [GRASS-dev] ascii export and import, large file problem

[Jerry Nelson]

I'm sorry to bug you again about this. Unless I'm doing something really
stupid (which is quite possible), both arcgis and grass do the same thing.
For a file whose x,y units are degrees (WGS84 ellipsoid), the slope tool
assumes the z units are also degrees. Here's a quote from the ESRI website
(http://webhelp.esri.com/arcgisdesktop/9.2/index.cfm?TopicName=Hillshade)

If your x,y units are decimal degrees and your z units are meters, some
appropriate z-factors for particular latitudes are:

    Latitude     Z-factor
     0           0.00000898
    10           0.00000912
    20           0.00000956
    30           0.00001036
    40           0.00001171
    50           0.00001395
    60           0.00001792
    70           0.00002619
    80           0.00005156

What I don't understand is that if you know lat and long, why can't the
software automatically figure out what the z-factor should be?

Jerry

-----Original Message-----
From: Glynn Clements [mailto:glynn at gclements.plus.com] 
Sent: Friday, April 13, 2007 9:52 AM
To: Gerald Nelson
Cc: Helena Mitasova; grass-dev list
Subject: Re: [GRASS-dev] ascii export and import, large file problem


Gerald Nelson wrote:

> Helena, I think grass assumes x,y, and z are all in the same units
> when it does the slope calculation. At least when I run r.slope.aspect
> on my lat long file I get junk out but when I project it to utm and
> then run r.slope.aspect I get numbers that look reasonable.

I get reasonable slope values out of r.slope.aspect for a lat/lon
location.

As Helena says, it uses geodesic distance for lat/lon locations (if
you haven't specified an ellipsoid, it defaults to WGS84).

-- 
Glynn Clements <glynn at gclements.plus.com>