[GRASS5] Projection question: Goode homolosine / m.proj2

[Glynn Clements]

Markus Neteler wrote:

> > > The lat/long boundaries are (stored into file 'latlong'):
> > > -9.1 47.3
> > > -9.1 -45.1
> > > 12.9 47.3
> > > 12.9 -45.1
> > > 
> > > RESOLUTION: 8000m
> > > 
> > > To get the boundaries in Goode projection, I used m.proj2 (enter in one
> > > line):
> > > 
> > > m.proj2 inproj="proj=ll" outproj="proj=goode,name=goode,
> > > ellps=wgs84,lat_0=0.0,lon_0=20.0,unfact=1.0"
> > > input=latlong output=goode
> > 
> > This doesn't give you the boundaries of the region. The point -9.1E 0N
> > is within the region but its projection will lie to the left of the
> > box obtained by projecting the corners.
> 
> Glynn, sorry for a naive question: How it is calculated properly?

I'm not entirely sure that I understand the question.

If you you are asking how to compute the bounding box in the Goode
projection of a lat/lon rectangle where the region straddles the
equator, you would need to include the corresponding segment of the
equator, as that will determine the left and/or right edges of the
projected region (in this case, just the left hand edge, as the entire
region is to the West of the central meridian).

E.g. feeding the following coordinates to m.proj2:

	12.9 47.3	# gives top, right
	-9.1 0.0	# gives left
	12.9 -45.1	# gives bottom (lon is irrelevant)

then taking the max/min x/y of the results should give the smallest
rectangle in the Goode projection which contains the source lat/lon
rectangle. Clearly, the resulting region will include points which lie
outside of the original lat/lon rectangle.

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