Converting Mapinfo Layers from WGS72 to WGS84

[NMZ]

Hi people,

Just wondering if someone knows the correct way in migrating my
MapInfo tables from WGS72 to WGS84. I tired saving copy as and then
change the spatial reference system to UTM 84 Zone 2 South but my
layers don't fall correctly on our new backdrop imagery which are in
UTM 84. I don't want to shift the layer which i was told is a big no
no.

Apperciate any help.

Thanks.

[Tuấn Vũ]

Hi All

Now In Vietnam have got a new projection with 7 parameter call VN2000 projection:
 -191,90441429 m;
 -39,30318279 m; 
-111,45032835 m.
 -0,00928836”
0,01975479”
-0,00427372”
k = 1,000000252906278

I Have a map with WGS84 projection.

I have modify in MAPINFO.PRJ are:
"Vietnam - VN2000"
"HCM - Binh Duong - Ben Tre - Long An - Tien Giang mui 6", 8, 9999, 28, -191.90441429, -39.30318279, -111.45032835, -0.00928836, 0.01975479, -0.00427372, -0.253, 0, 7, 105.75, 0, 0.9996, 500000, 0 for 6 degree

But i don't know that is it exactly or not?

When I convert form wgs84 to VN2000 with way save copy as with vn2000 projection.

How to covert from wgs84 to Vn2000 in MapInfo?

Thanks

[David R Sherrod]

The letter from Nome at NMZ came in nearly a week ago, then was trumped in reply by a change in theme.  That seems to have left Nome's original question hanging fire.

I am interested in any possible solutions but uncertain myself of how to address the problem.  I'll offer some ideas and see what bounces back from MI-L.

Nome is working with QuickBird imagery for Samoa, where a traditional projection is the WSIG--Western Samoa Integrated Grid WGS72.  The QuickBird imagery is in UTM zone 2 S, WGS84, and the task is to reproject Mapinfo Tables from WSIG into the projection of the QuickBird data.  Efforts produce visual shifts so that vector files lie about 12 m west and 3 m south of where they should be from visual inspection.
"WSIG", 8, 103, 7, -172, 0, 1.0000, 700000, 7000000" .

Three comments from me.
1.  QuickBird data can have geographic problems of its own.  A "best effort" is made to get everything registered correctly, but the only way to know the quality of the resulting imagery, once projected, is to superimpose data of known status--Benchmarks, buildings, roads, pipelines, whatever.  It wouldn't surprise me to learn that QuickBird registration problems might be greatest in geographically isolated areas--oceanic islands being a supreme example.  Skepticism of imposed data is always warranted if you trust your own stuff.

2.  Nome's problem sounds somewhat like a datum shift, which allows the possibility that the vector data was projected incorrectly in the first place.  Any efforts to simply save the table to a new projection (save copy as, with new projection selected within the options there) cannot overcome errors in the source data.  But the direction of shift described doesn't match my expectations for what might happen when switching between the two ellipsoids.

3.  A verification of the projection parameters in the vector data could be made by exporting the file to mid/mif format and then opening the mif version with a text teditor.  The projection code and bounds will be listed therein.

4.  When I wish to be scrupulous to avoid any possible source of error on my part, I close all raster images, then use the map window (MAP-->Options-->Projection) to reproject into my desired datum and projection, then use the Mapbasic window to insert the command "Set Coordsys Window FrontWindow()".  Then I "save copy as," using the projection button to select my new projection.  I expect that others on this list will tell me I'm exerting needless effort when I undertake the map-window reprojection step.  But overkill isn't always a bad thing.


Dave Sherrod
-----------------------------------------

From:	NMZ <nome....@gmail.com>
To:	MapInfo-L <mapi...@googlegroups.com>
Date:	12/02/2009 10:03 PM
Subject:	[MI-L] Converting Mapinfo Layers from WGS72 to WGS84

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