[GRASSLIST:1707] Re: v.proj bug?

[David Orme]

Hamish - thanks for the quick (and authoritative!) suggestions - I'm  
working with a 1982 atlas of New Zealand bird distributions mapped at a  
10,000yd resolution on the old SI/NI grids.

I've run those test points through concord and get something very  
similar (but not identical to) the values from NZPointConverter   
(http://www.ualberta.ca/~norris/gps/nz/NZPointConverter.SIT.hqx) - they  
also match closely to a printed grid and ArcMap:

LatLong
166 27 28.0 E 46 00 55.0 S Cape_Providence
173 01 05.1 E 40 33 28.8 S Farewell_Spit

NZPointConverter
72976 E 241337 N Cape_Providence
640603 E 916716 N Farewell_Spit

Concord
72974.8 E 241330.6 N Cape_Providence
640602.0 E 916716.1 N Farewell_Spit

So it looks certain that [GRASS] / [my usage of GRASS and cs2cs]  
(delete as applicable) is going astray.

To be clear about the origin of the test points - these are coordinates  
taken from a downloaded DCW e00; the PRJ section identifies a Clarke  
1866 spheroid but no datum - I think ArcMAP imposes an assumed NAD27  
datum. I can't get cs2cs to convert NAD27 to NZGD49 (using the datum  
grid definition Hamish referred to):

> [doibook:~] dorme% cs2cs +proj=latlong +datum=NAD27 +to +proj=tmerc  
> +ellps=intl +nadgrids=nzgd2kgrid9911.asc +units=yd +lat_0=44S  
> +lon_0=171.5E +x_0=500000 +y_0=500000
> 166d27'28.0E 46d00'55.0S
> 173d01'05.1E 40d33'28.8S
> *       * 0.00
> *       * 0.00

If I just specify the NAD27 ellipsoid (clarke 1866) and shift to the  
South Island grid using NZGD49 (rather than just the international  
spheroid as before) then the point are no closer to the reference  
values above from concord:

> [doibook:~] dorme% cs2cs +proj=latlong +ellps=clrk66 +to +proj=tmerc  
> +ellps=intl +towgs84=54.4,-20.1,183.1 +units=yd +lat_0=44S  
> +lon_0=171.5E +x_0=500000 +y_0=500000
> 166d27'28.0E 46d00'55.0S
> 173d01'05.1E 40d33'28.8S
> 119762.12       288494.13 -257.34
> 687428.17       963890.56 -243.43

There seems to be a ~45,000 yard difference - changing the position of  
the units argument leads to differences of a similar order of magnitude.

Cheers,
David

On Friday, November 7, 2003, at 08:09  am, Hamish wrote:

>> I have a raster dataset in the old New Zealand South Island grid
>> (Transverse Mercator: central meridian 171.5E, latitude of origin 44S,
>> scale 1, false easting 500000, false northing 500000, units yards,
>> ellipse international). This projection is included in ArcMAP and when
>> I compare a graticuled map in ArcMAP to (coarse scaled) printed grid
>> of the raster  for the dataset, there is a very close match.
>
> I have this definintion (from "concord"):
>
> SNAT "South Island National Grid (yards)" REF_FRAME NZGD1949 &
>           PROJECTION TM 171.5 -44 1.00000 500000.0 500000.0 0.91439841  
> &
>           RANGE 0 0 800000 1000000
>
>
> No time to look into this right now, but you might compare against what
> "concord" gives: [runs fine under wine]
>
> http://www.linz.govt.nz/rcs/linz/pub/web/root/core/SurveySystem/ 
> GeodeticInfo/Software/snapdownload/index.jsp
>
> That "should" give a correct answer....
>
>
>
> ** Check if you are using the correct parameters for the NZGD49 datum:
> [use GRASS 5.3+]
>
> src/libes/gis/datum.table
> src/libes/gis/datumtransform.table
> ----------------------------------
>
> http://www.linz.govt.nz/rcs/linz/pub/web/root/core/SurveySystem/ 
> GeodeticInfo/GeodeticDatums/nzgd49tonzgd2000/index.jsp
> nzgd49 "New_Zealand_Geodetic_Datum_1949" international dx=54.4   
> dy=-20.1  dz=183.1
>
>
> http://www.linz.govt.nz/rcs/linz/pub/web/root/core/SurveySystem/ 
> GeodeticInfo/Software/snapdownload/index.jsp
> nzgd49 "nadgrids=nzgd2kgrid0005.gsb" "New Zealand" "LINZ NZGD49 NTv2  
> Distortion Model, accuracy 10-30cm"
>
>
> http://www.linz.govt.nz/rcs/linz/6149/osgpol97_3.pdf
> nzgd49 "towgs84=59.47,-5.04,187.44,0.47,-0.1,1.024,-4.5993" "New  
> Zealand" "Accuracy approx. 4m"
>
>
> I'll get around to looking into this at some point, but can't now.
>
> Curious to know what the data is & where it's from..
>
>
>
> regards,
> Hamish
> (way way down on the South Island, New Zealand)
>