Wed, Jul 19 2006
09:28:20
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Request created by hbowman
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Subject: d.grid -g, ps.map: incorrect geogrid rendering
Hi,
"d.grid -g" and ps.map's geogrid command are not calculating the correct grid
lines for me. (lines are drawn 90m to the north)
Perhaps it is only doing the projection, and not the datum transform ?!!?
Lon is *exactly* correct though, only lat is just slightly 3" to the north. Weird.
"d.where -w" gets it correct. Also tested with a v.mkproj in LL location +
v.proj into projected location. So I am pretty happy the data is ok just the
grid lines are drawn wrong.
Happens with both NTv2 grid and 7-term transform parms,
proj=nzmg
datum=nzgd49
?
Hamish
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Wed, Jul 19 2006
11:01:28
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Re: [GRASS-dev] [bug #4877] (grass) d.grid -g, ps.map: incorrect geogrid rendering
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Hello Hamish
On Wed, 19 Jul 2006, Request Tracker wrote:
> this bug's URL: http://intevation.de/rt/webrt?serial_num=4877
> -------------------------------------------------------------------------
>
> Subject: d.grid -g, ps.map: incorrect geogrid rendering
>
> Hi,
>
> "d.grid -g" and ps.map's geogrid command are not calculating the correct grid
> lines for me. (lines are drawn 90m to the north)
>
> Perhaps it is only doing the projection, and not the datum transform ?!!?
Looking at the code, this seems quite possible. I have applied a patch in
CVS - can you test? If it's working now, I expect we should be able to
apply a similar patch to ps.map.
Slightly more in-depth explanation:
this behaviour stems from a questionable "feature" of PROJ (my words
really although Frank has agreed too---see PROJ bug 368
http://bugzilla.remotesensing.org/show_bug.cgi?id=368 ) where if one
co-ordinate system has datum transformation parameters specified and
the other does not, a kind of partial transformation is done. IMHO if the
datum information is not fully specified it should be ignored completely,
but that is not possible the way the datum information is contained as
part of the co-ordinate system in PROJ. Thierry Laronde and KerGIS were
doing this a different way to avoid this problem as far as I remember...
Paul
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Wed, Jul 19 2006
11:57:50
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Wed, 19 Jul 2006 21:57:45 +1200
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Subject |
Re: [bug #4877] (grass) d.grid -g, ps.map: incorrect
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> > this bug's URL: http://intevation.de/rt/webrt?serial_num=4877
> > -------------------------------------------------------------------
..
> > "d.grid -g" and ps.map's geogrid command are not calculating the
> > correct grid lines for me. (lines are drawn 90m to the north)
> >
> > Perhaps it is only doing the projection, and not the datum transform
> > ?!!?
>
> Looking at the code, this seems quite possible. I have applied a patch
> in CVS - can you test? If it's working now, I expect we should be
> able to apply a similar patch to ps.map.
umm, now it is twice as bad. the 'd.grid -g' line is now drawn 180m
north of the expected location.
> Slightly more in-depth explanation:
> this behaviour stems from a questionable "feature" of PROJ (my words
> really although Frank has agreed too---see PROJ bug 368
> http://bugzilla.remotesensing.org/show_bug.cgi?id=368 ) where if one
> co-ordinate system has datum transformation parameters specified and
> the other does not, a kind of partial transformation is done. IMHO if
> the datum information is not fully specified it should be ignored
> completely, but that is not possible the way the datum information is
> contained as part of the co-ordinate system in PROJ. Thierry Laronde
> and KerGIS were doing this a different way to avoid this problem as
> far as I remember...
I take it that in this case the LL datum was the one missing?
#rendering location
G61 > g.proj -p
-PROJ_INFO-------------------------------------------------
name : New Zealand Map Grid
datum : nzgd49
nadgrids : nzgd2kgrid0005.gsb
proj : nzmg
ellps : international
a : 6378388.0000000000
es : 0.0067226700
f : 297.0000000000
lat_0 : -41.0000000000
lon_0 : 173.0000000000
x_0 : 2510000.0000000000
y_0 : 6023150.0000000000
-PROJ_UNITS------------------------------------------------
unit : meter
units : meters
meters : 1.0
Hamish
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Wed, Jul 19 2006
13:16:41
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Re: [bug #4877] (grass) d.grid -g, ps.map: incorrect
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On Wed, 19 Jul 2006, Hamish wrote:
>>> this bug's URL: http://intevation.de/rt/webrt?serial_num=4877
>>> -------------------------------------------------------------------
> ..
>>> "d.grid -g" and ps.map's geogrid command are not calculating the
>>> correct grid lines for me. (lines are drawn 90m to the north)
>>>
>>> Perhaps it is only doing the projection, and not the datum transform
>>> ?!!?
>>
>> Looking at the code, this seems quite possible. I have applied a patch
>> in CVS - can you test? If it's working now, I expect we should be
>> able to apply a similar patch to ps.map.
>
> umm, now it is twice as bad. the 'd.grid -g' line is now drawn 180m
> north of the expected location.
For me (I've tried with a test NZMG/nzgd49 location) before the patch the
lines were about 90m *south* of where they'd be expected to be from the
d.where -l output, and after the patch they agree :/ d.grid is supposed to
show lines of latitude&longtiude on the current ellipsoid, not WGS84,
right?
> I take it that in this case the LL datum was the one missing?
Yes the way it was being done was ll was set as the projection and the
ellipsoid parameters were copied over to the new parameters, but not the
datum. I've double-checked and am fairly confident that particular problem
is fixed now, but perhaps there was something else in the code that
compensated for this bug and so something else needs changed? I'm not sure
where to look.
Paul
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Wed, Jul 19 2006
13:56:21
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Wed, 19 Jul 2006 23:56:14 +1200
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Re: [bug #4877] (grass) d.grid -g, ps.map: incorrect
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> >>> this bug's URL: http://intevation.de/rt/webrt?serial_num=4877
> >-------------------------------------------------------------------
> > ..
> >>> "d.grid -g" and ps.map's geogrid command are not calculating the
> >>> correct grid lines for me. (lines are drawn 90m to the north)
..
> > umm, now it is twice as bad. the 'd.grid -g' line is now drawn 180m
> > north of the expected location.
>
> For me (I've tried with a test NZMG/nzgd49 location) before the patch
> the lines were about 90m *south* of where they'd be expected to be
> from the d.where -l output, and after the patch they agree :/ d.grid
> is supposed to show lines of latitude&longtiude on the current
> ellipsoid, not WGS84, right?
aah, ok, that's it. The nzgd49 datum uses the int'l ellipsoid.
using "d.where -l" I see that "d.grid -g" does in fact lie in the
correct place (after applying your patch).
the rasterized nautical chart I was using as a backdrop is WGS84.
using "d.where -w" the printed rasterized grid lines match up.
I guess d.grid's default should be to use the current ellipsoid; should
d.grid get a -w flag too?
thanks,
Hamish
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Wed, Jul 19 2006
16:13:51
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Re: [bug #4877] (grass) d.grid -g, ps.map: incorrect
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On Wed, 19 Jul 2006, Hamish wrote:
> the rasterized nautical chart I was using as a backdrop is WGS84.
> using "d.where -w" the printed rasterized grid lines match up.
>
> I guess d.grid's default should be to use the current ellipsoid; should
> d.grid get a -w flag too?
I don't see why not. I have added this functionality to the CVS HEAD
version, and backported the bugfixes to d.grid and ps.map to the 6.1
release branch.
Paul
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04:14:03
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Re: [bug #4877] (grass) d.grid -g, ps.map: incorrect
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> > the rasterized nautical chart I was using as a backdrop is WGS84.
> > using "d.where -w" the printed rasterized grid lines match up.
> >
> > I guess d.grid's default should be to use the current ellipsoid;
> > should d.grid get a -w flag too?
>
> I don't see why not. I have added this functionality to the CVS HEAD
> version, and backported the bugfixes to d.grid and ps.map to the 6.1
> release branch.
-w works a treat, thanks
Hamish
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Tue, Jul 25 2006
12:57:27
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Status changed to resolved by pkelly
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