[GRASSLIST:2529] Re: Which projection?

Eric G. Miller egm2 at jps.net
Fri Sep 28 20:48:38 EDT 2001


On Fri, 28 Sep 2001 16:05:50 +1000, Gordon Keith <gordon.keith at marine.csiro.au> wrote:

[snip]
> I can get the same data output using a different coordinate system with 
> the header:
> 
> Datum: WGS84
> Half axis: 6378137.0000000
> Flattening: 1/298.25722356300
> Coordinate system: merc
> Y min.:  -5248244.68
> X min.:   4966111.48
> Y max.:  -5219451.16
> X max.:   4985106.57
> Latitude cell size: 30.00 meter
> Longitude cell size: 30.00 meter

I suggest using the mercator rather than lat/lon as the translation to cell
dimensions is much easier.

> I'm not very familar with projections and don't understand the 
> significance of the half axis and flattening terms.

"Half axis" is probably equivalent to the "a" term for an ellipsoid, or
the radius of the Earth at the equator, aka "semi-major axis".  The 
flattening ratio simply defines how much to squash the ellipsoid as
you move north or south from the equator.  WGS84 as a datum is
enough to specify all these parameters by reference.


> The data is gridded, but the grid coordinates don't follow lines of 
> latitude and longitude, so I haven't had good results reading the data 
> into grass using a latitude longitude projection.
> 
> Could someone please suggest the projection and information I will need 
> to create it in grass?
> 

Well, it's probably guesswork unless you have more information.  Mercator
wants latitude of true scale, central meridian and a scale factor.  You
could assume 0,0,1.0 -- but that might not be correct..

> I will need to work with this data in latitude/longitude coordinates. 
> Does this mean I will have to reproject (r.proj) the data into a 
> latitude longitude projection, if I get it into grass successfully? 

Yes. Unless you can figure out appropriate cell dimensions as fractions
of degrees.  If the data is really referenced to lat/lon already, then
this might be possible.  Seems you weren't given much info though...

Luck,
-- 
Eric G. Miller <egm2 at jps.net>



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