[GRASSLIST:4325] Re: GIS visitor requests directions

[Glynn Clements]

Nigel McFarlane wrote:

> When I created a second location based on the Universal
> Polar Stereographic projection, ups, (one that should merely
> makes the re-projection results obvious), I found I don't
> understand the concept of 'region' or 'region coordinates'.
> 
> In a lat-long projection I can enter the raw lat-longs,
> but most other projections require these largish numbers
> whose origin I don't know. That left me with the
> default of 1, 0, 1, 0 for the ups projection, and an
> r.proj operation failed:
> 
>    (Central latitude 70N, central meridian 135W)
> 
>    r.proj input=ustopo location=PERMANENT mapset=testmap output=outfile
> 
>    ERROR: Input map is outside current region
> 
> I happened to look at UTM (which I don't need) and now I
> understand a bit about Eastings and Northings, but I don't
> understand how to calculate them for other projections,
> or even how Eastings or Northings could apply to other
> projections, especially a polar one. How can you be East
> of the north pole?

"East" corresponds to the positive X axis, "North" corresponds to the
positive Y axis. Most projections (e.g. those which don't include
either pole, or for which poles correspond to the edges) are oriented
accordingly (i.e. North is up).

> - where do these numbers come from?

Eastings and Northings are basically the X,Y coordinates on the
projected map. Their dimensions are physical units, (e.g. metres,
international feet or US survey feet); the origin depends upon the
projection.

There is usually a "natural" origin, determined by the nature of the
projection (e.g. for UPS this would be at the pole). However, most
projections use a false Easting and/or Northing (fixed values which
are added to the coordinates), so that coordinates are positive (e.g. 
UPS has false Easting and Northing of 2000000).

So, for the default parameters, UPS has the Y axis parallel to the 0 &
180 meridians, the X axis parallel to the 90E & 90W meridians, the
origin such that the North pole is at (2000000E,2000000N), with units
of metres.

You can get a rough idea of the appropriate region boundaries by
projecting a sample set of lat-lon coordinates into the target
projection using m.proj.

> - is a region merely a pixellated subsection of a full projection?

A GRASS "region" is a rectangle, in the projection's coordinate
system, along with E-W and N-S (IOW, X and Y) resolutions. This is
used primarily for raster maps.

Each map has its own region, which reflects the correspondence between
pixel array indices and geographic coordinates. However, most GRASS
programs automatically resample input raster maps according to the
current region, and any generate maps will normally have the
boundaries and resolution dictated by the current region.

> - must the two projections (source and target) somehow 'fit'
>    each other? (for example, must one contain the other?)

r.proj requires that the projection of the source region intersects
the destination region, otherwise it generates the error which you
report. This normally indicates that the destination region is
incorrect.

-- 
Glynn Clements <glynn.clements at virgin.net>