[GRASSLIST:1621] Re: More projection problems...

[Morten Hulden]

On Fri, 16 Mar 2001, mberglund wrote:

> After trying what Morten suggested, (shifting the false easting) yes it is
> closer, but still  about 8000 feet off.

Yes, you can keep changing the parameters until you get the points to
match. But one test point is not enough if you want to try to match the
maps by altering the parameters. You need many points with known
co-ordinates all over the map. For this kind of matching i.rectify maybe
is a better tool. I just mentioned reducing the false easting as one way
of adjusting the Albers map.

The main question still is: Are you sure that the upper left corner
co-ordinates of the two locations really should coinside. Can you locate a
geographic feature (city, lake, whatever) for which you know the
co-ordinates, and which is present on both maps? If you can, then use that
as a testpoint. 

> Is this what you meant by the "expecting" too much from the albers proj?

What I meant was: Your STP location is in 1-foot resolution (1x1 foot
cells), the Albers map is in 1-meter resolution. You cannot expect to get
more detail than what is available in the Albers map. You might as well
reduce the resolution of the STP location temporarily to about 3x3 feet
cell size before importing the Albers map (by defining a region within the
STP location, with the same borders as the default region but with a
different resolution). The projection will be faster and the imported map
will become smaller in size, but the quality will still be the same.

Also, I wanted raise the question whether your Albers map really _is_ in
1-meter resolution. Or did you just create the location that way? I
believe it's unusual to have Albers in such high resolution, but of course
it is possible.

Sorry if I was unclear. It was 3 am before I got my previous response
together.

> I will keep plugging here, if he can do the conversion in ARC/Info, grass
> has to be able to do it!

Grass can do it better. It's only a matter of getting the data parameters
right.

regards
Morten