[GRASSLIST:7957] Re: nad27/nad83 datum conversion for canada using Ntv2

GRASS GRASS user
1

---------- Forwarded Message ----------

Subject: Re: [GRASSLIST:7944] nad27/nad83 datum conversion for canada using
Ntv2
Date: Wednesday 17 August 2005 13:46
From: Feng Tan <feng@lynxseismicdata.com>
To: Feng Tan <feng@lynxseismicdata.com>

Hi, Listers,

I just did some testing about nad27/nad83 datum conversion for a point in
Alberta, Canada using GRASS6.0 comparing with ESRI ARCGIS9.1, following is
the result, it has about +-100m difference with or without NTV2_0.GSB. I wish
GRASS can implement newer version of Canadian National Transformation
NTV2_0.GSB nad27/nad83 datum shift grid file soon, so we can use GRASS more
confidently. Right now GRASS6.0 still apply old ntv1_can.dat datum shift
file. I think this is very important issue and worth to put some effort in.

Here is the test:

I took 1 point in UTM coordinate system, nad27 datum and converted it to UTM
nad83 from GRASS6.0 and ESRI ArcGIS9.1

UTM_nad27: easting 889298.45 northing 8682992.65

1.) ArcGIS9.1 using NTv2_0.GSB datum shift grid file
UTM_nad83: easting 889220.78 northing 8683069.16

2) GRASS6.0 cs2cs function using ntv1_can.dat datum shift file
UTM_nad83: easting 889200.13 northing 8683206.65

3) GRASS6.0 cs2cs function without applying ntv1_can datum shift file
UTM_nad83: easting 889277.91 northing 8683130.97

Please help!!

Thanks,

Feng

On Wednesday 17 August 2005 08:49, you wrote:

Hi, List,

1.) I want to do datum conversion for Canada using Ntv2 grid file, how
should I set up the locations for them? It seems grass6.0 only use
Ntv1.data grid to set nad27 datum for canada (no option to use Ntv1.dat for
nad83 datum), but Ntv2 version is more accurate grid shife file, how can I
apply Ntv2 grid file for both nad27 and nad83 locations?

2.) If I use cs2cs to do the conversion for canada, what is the procedure
for applying Canadian NTv2.gsb grid file?

3.) Did anybody evaluate the converted coordinate accuracy between last 2
methods in GRASS6.0, or against other softwares such as ESRI and FME? What
the outcome? Do they get the same results?

Thanks,

Feng

-------------------------------------------------------

2

On Wed, 17 Aug 2005, Feng Tan wrote:

I just did some testing about nad27/nad83 datum conversion for a point in
Alberta, Canada using GRASS6.0 comparing with ESRI ARCGIS9.1, following is
the result, it has about +-100m difference with or without NTV2_0.GSB. I wish
GRASS can implement newer version of Canadian National Transformation
NTV2_0.GSB nad27/nad83 datum shift grid file soon, so we can use GRASS more
confidently.

GRASS (and PROJ.4) *can* use NTv2 GSB (binary) files and has been able to since the datum shifting support was added in 5.3. I'm not sure what your problem is here. What are you trying to change?
Put the gsb file in the $GISBASE/etc/nad directory and edit the PROJ_INFO file for your NAD27 location to point to the new file.

Paul

3

Hi, Paul,

Thank you very much for the answer. I went to $GISBASE/etc edited
datumtransform.table and replaced ntv1_can.dat with NTV2_0.GSB, I start a new
location by chosing NTV2_0.GSB, but cs2cs failed. I did following

IN_PROJ="+proj=utm +zone=10 +a=6378206.4 +rf=294.9786982 +no_defs

+nadgrids="$GISBASE/etc/nad/NTV2_0.GSB""

OUT_PROJ="+proj=utm +zone=10 +a=6378137 +rf=298.257222101 +no_defs

+towgs84=0.000,0.000,0.000"

echo "889298.45 8682992.65" |cs2cs -f %.7f $IN_PROJ +to $OUT_PROJ

* * 0.0000000

what is wrong???

Thanks,

Feng

On Wednesday 17 August 2005 14:22, you wrote:

On Wed, 17 Aug 2005, Feng Tan wrote:
> I just did some testing about nad27/nad83 datum conversion for a point in
> Alberta, Canada using GRASS6.0 comparing with ESRI ARCGIS9.1, following
> is the result, it has about +-100m difference with or without NTV2_0.GSB.
> I wish GRASS can implement newer version of Canadian National
> Transformation NTV2_0.GSB nad27/nad83 datum shift grid file soon, so we
> can use GRASS more confidently.

GRASS (and PROJ.4) *can* use NTv2 GSB (binary) files and has been able to
since the datum shifting support was added in 5.3. I'm not sure what
your problem is here. What are you trying to change?
Put the gsb file in the $GISBASE/etc/nad directory and edit the PROJ_INFO
file for your NAD27 location to point to the new file.

Paul