[GRASS-user] ascii files to import

[Michael Barton]

Kirk,


On 7/31/07 8:34 AM, "Kirk Wythers" <kwythers at umn.edu> wrote:

> 
> On Jul 31, 2007, at 9:47 AM, Michael Barton wrote:
> 
>> Kirk,
>> 
>> Your method #1 is straightforward and should go fast. Do you need all 100
>> columns?
> 
> Thanks Michael. Yes, I am afraid that I need all 100, if only because I am not
> sure that I won't someday! 
> 
>> 
>> A 3rd way is to make sure that the file has a column to serve as a
>> "category/cat" field (integer), save it as a dbf file from Excel and use
>> v.in.db to create the points. This has the advantage that you can name the
>> fields more easily. You would then need to reproject it into UTM. 
> 
> With this approach how would reproject to UTM? I assume you have read the dbf
> file into a lat lon location. Right? However, then I would need to move the
> data from the lat lon location to the UTM location.
> 
Use v.proj. This would be the case with any GRASS method.

Start in latlon location, import points, switch to utm location (g.mapset or
just exit and restart in utm location), reproject using v.proj.
> 
>> 
>> I'd be a little nervous about trying to convert from latlon to UTM within a
>> database. GRASS has routines specially designed to do accurate reprojection.
> 
> This actually works pretty well using the PostGIS extensions on PostgrSQL (you
> can use db.connect to point grass to a pg database rather than the default
> dbf). There is a very useful tutorial written by Regina Obe´
> 
> http://www.bostongis.com/?content_name=postgis_tut03#30
> 
I took a brief look at the tutorial. I may have missed it, but didn't see
anything about where you would set the datum, ellipsoid, and zone for the
UTM projection you want. The tutorial only mentions WGS84. These settings
can make a significant difference. e.g., here in Arizona, the difference
between using NAD27 and NAD83 can be over 200m, and can put a point on the
wrong side of a road or canyon. If you have set up your UTM location
correctly, GRASS and v.proj will take this kind of important information
into account when setting new point coordinates.

Michael
__________________________________________
Michael Barton, Professor of Anthropology
Director of Graduate Studies
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton


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