[GRASS-user] Re: grass-user Digest, Vol 49, Issue 27

[Kurt Springs]

Thanks Rich and Dylan

I downloaded the pdf of document #1395.  At the moment I am leaning toward Lambert Conic Conformal (1SP) since it seems to use Lat/Long of Natural Origin, in case I need to use a GPS.  If I am reading you right  Latitude and longitude don't even come into the equation, just the projection.

I've been looking at the website http://www.dmap.co.uk/utmworld.htm.  I was mistaken it was 18 and 19T that NH falls in.  However, it appears to be just the western most sliver.  However, if I don't have to figure out the conversion, so much the better.

Kurt
On May 14, 2010, at 12:00 PM, grass-user-request at lists.osgeo.org wrote:

> 
> Message: 2
> Date: Fri, 14 May 2010 08:10:20 -0700
> From: Dylan Beaudette <dylan.beaudette at gmail.com>
> Subject: Re: [GRASS-user] Latitude/Longitude vs UTM
> To: Rich Shepard <rshepard at appl-ecosys.com>
> Cc: GRASS user list <grass-user at lists.osgeo.org>
> Message-ID:
> 	<AANLkTimnbx-ED-oCXMdGTm_Pw5ngzM48CbabS98twsF4 at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
> 
> On Fri, May 14, 2010 at 6:22 AM, Rich Shepard <rshepard at appl-ecosys.com> wrote:
>> On Thu, 13 May 2010, Kurt Springs wrote:
>> 
>>> This was interesting in that it told me that r.topidx could not be run
>>> with latitude and longitude and I had to convert to UTM. I was wondering
>>> if this is the answer to the problem and I just had to convert to UTM.
>> 
>> Kurt,
>> 
>>  Lat/Long represents geographic coordinates, not a projection of location
>> on a mathematial model of the earth. UTM is the Universal Transverse
>> Mercador projection that we see on most printed (or computer displayed) maps
>> of the earth. There is documentation within the GRASS Web site that provides
>> a good explanation of the differences. GRASS modules work on geographic
>> projections, not just coordinates.
>> 
>>  There is a USGS technical report from the mid-1980s that's the standard on
>> projections. While it is becoming more rare to locatate, see if you can find
>> a copy.
> 
> I think that Rich is referring to this USGS document, #1395
> 
> http://pubs.er.usgs.gov/usgspubs/pp/pp1395
> 
> Definitely worth the price if you want to become an expert in map projections.
> 
> 
>>> One other question. New Hampshire appears to fall within two UTM zones
>>> (19T and 20T) Is there a way for a maps set to contain two UTM zones?
> 
> Yes. Don't use UTM. In this case use a regional projection that suits
> your needs:
> 
> 1) navigation --> use a conformal projection
> 2) area statistics --> use an equal-area projection
> ... etc ...
> 
> Variations on the Albers or Lambert (conformal) conic projections work
> quite well for large regions that are wider than tall, but for such as
> small state should be just fine. We use an Albers equal-area
> projection to house soil survey data for the lower 48 states.
> 
>>  Interesting. NH is a tall, narrow state so one would assume it would be
>> within a single zone. Regardless, yes there is a way to reproject locations
>> in one zone on the other, but it's non trivial and I've not done it.
> 
> I wouldn't recommend it. The desirable properties of the UTM system
> (i.e. the fairly good compromise between distortion, preservation of
> angles, and preservation of area) only occur within a zone's
> boundaries. The farther you move from the central meridian of the UTM
> zone, the more distortion you will encounter-- therefore 'projecting'
> UTM z10 data into UTM z11 is technically possible, but not a great
> idea.
> 
>>  Oregon is primarily in Zone 10, but the eastern edge (I don't recall the
>> distance within the state) is in Zone 11. The available DEM and hydrologic
>> data were reprojected from 11 to 10 by the supplying agency.
> 
> Hmm...
> 
> Dylan
> 
>> Rich
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