[GRASS5] Datum transformation with r.proj

GRASS GIS GRASS GIS developer
1

Hi,

I was curious about whether r.proj and v.proj automatically do datum transformation
if it seems necessary. I followed the output and it seems to be doing it:

Input Projection Parameters: +proj=nzmg +lat_0=-41.0000000000 +lon_0=173.0000000000 +x_0=2510000.0000000000 +y_0=6023150.0000000000 +a=6378388 +rf=297 +no_defs +nadgrids=nzgd2kgrid0005.gsb
Input Unit Factor: 1

Output Projection Parameters: +proj=tmerc +lat_0=0.0000000000 +lon_0=173.0000000000 +k_0=0.9996000000 +x_0=1600000.0000000000 +y_0=10000000.0000000000 +a=6378137 +rf=298.257222101 +no_defs +towgs84=0.000,0.000,0.000
Output Unit Factor: 1
Input:
Cols: 45 (10200)
Rows: 50 (14900)
North: 5444600.000000 (6790000.000000)
South: 5439600.000000 (5300000.000000)
West: 2006400.000000 (1990000.000000)
East: 2010900.000000 (3010000.000000)
ew-res: 100.000000
ns-res: 100.000000

Output:
Cols: 41 (41)
Rows: 46 (46)
North: 4882159.892804 (4882159.892804)
South: 4877561.531759 (4877561.531759)
West: 1096524.283564 (1096524.283564)
East: 1100623.252896 (1100623.252896)
ew-res: 99.974862
ns-res: 99.964371
Allocating memory and reading input map... 100%
Projecting... 100%

-----------
Both locations have their projections and datums set correctly using
entries already in the etc/datumtransform.table file.

However, when I check the my maps, they are still off centre by the
amount expected by no datum transformation....

(Note, perhaps the man page for r.proj should be changed to
mention that it automatically does datum transformation,
rather than "supporting" it. Since I wasn't sure if there were
meant to be some missing parameters or what....)

Any help appreciated, thanks
Joel

2

Hi again,

Sorry, I just realised that it is most likely my data, and it is
actually off by ~5x the expected distance without datum
transformation.

Joel Peter William Pitt wrote:

Hi,

I was curious about whether r.proj and v.proj automatically do datum transformation
if it seems necessary. I followed the output and it seems to be doing it:

<snip>

3

On Tue, 5 Oct 2004, Joel Peter William Pitt wrote:

(Note, perhaps the man page for r.proj should be changed to
mention that it automatically does datum transformation,
rather than "supporting" it. Since I wasn't sure if there were
meant to be some missing parameters or what....)

It 'supports' it in the sense that if the projection information files for both locations define a datum, then a datum transformation will take place. But there is no easy way for r.proj to know whether a datum transformation is happening or not---it all takes place inside the PROJ.4
library.

You can use 'g.proj -d' (in 5.7) to verify if a location supports datum transformation (i.e. has a datum correctly defined).

Paul K