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

[MS]

Is projecting to NH state plane and option? Along the lines of using  
something more localized.

Mark

On May 14, 2010, at 2:06 PM, Kurt Springs <ferret_bard at mac.com> wrote:

> 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
>>> _______________________________________________
>>> grass-user mailing list
>>> grass-user at lists.osgeo.org
>>> http://lists.osgeo.org/mailman/listinfo/grass-user
>
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