[GRASSLIST:4174] Re: Map import cluelessness

[C.S. Cornuelle]

Ian,

See below, please.

Thus spake Ian MacMillan (ian_macmillan at umail.ucsb.edu):

> So as I understand it, I think you set up the mercator projection 
> correctly.  One question, are you sure you want a mercator projection, 
> and not a UTM (universal transverse mercator) projection?  Generally 
> mercator projections are only used with large areas of the world.  With 
> a UTM, you just need to input the zone number, a map can be found here.
> http://www.dmap.co.uk/utmworld.htm

Yes, I need mercator as the map is in that projection and covers 
Eurasia and Africa.  Too big, I agree.  :^/

> If you do want a mercator projection, then the typical parameters for a 
> world map would be pretty much what you put
> Latitude of true scale : 0
> Cenral meridian : whatever is appropriate for your map
> Scale factor at central meridian : 1
> plural form of units : meters
> 
> This location differs from an XY location in that as you move away from 
> the latitude of true scale, the vertical and horizontal scale do not 
> stay equal to each other.  For example, world mercator maps really 
> distort the north and south poles.  An xy location works essentially 
> like Photoshop, the x axis and y axis are always the same scale.
> 
> From what I know, all types of reprojecting like you are trying can be 
> done completely within GRASS.
> 
> Does this help?

Hmm.  I'm trying eventually to get to lat-lon.  As you nicely (thanks 
again) describe it, it is necessary to read the file into an XY
location, rectify it (i.rectify and related commands) into a Mercator 
location, and finally import it into a lat-lon location.  

All well and good.  However, the warning in i.rectify where the
projections do not match (well, of course they don't ...) is still
there.  This in fact leads to null maps on the output for me.

What confuses me more is that once the Mercator location is established,
setting the projection seems only to leave the option of making the map
the same size and resolution as the corresponding XY location.  It does
not seem possible to enter into DEFINE THE DEFAULT REGION any degree-
based coordinates, e.g. 71N and such.  If the result is to be a fully
geocoded database, then I would have guessed that the Mercator location
would be defined in terms of longitude and latitude.

Clearly, again, I'm missing something here ...

<snip>

-- 
Adios,
Chris Cornuelle
bob at xmission dot com