Re-projection on the fly

[Frank Warmerdam]

On 8/18/05, Krung Saengpole <saengpole at yahoo.com> wrote:
> Hi all, 
> I have one trouble about projection. My testing area is around UTM zone-47
> and zone-48. The official road shapefile is translated both zones into
> zone-47 (Indian 1975). My road is in zone-48 (WGS84). So, I'd like to
> translate my utm-48 road data into zone-47. I used: 
>   
> $proj48=ms_newProjectionObj("proj=utm,zone=48,ellps=WGS84,datum=WGS84,units=m,no_defs");
> $proj47=ms_newProjectionObj("proj=utm,zone=47,a=6377276.345,b=6356075.413,towgs84=209,818,290,0,0,0,0,units=m,no_defs");

> My road should overlay onto official road. But it wasn't on the right place,
> it shifted to the left of official road about 350m. If I use data by not
> translating in map file as: 

Krung,

The error you quote sounds typical of a missing or misapplied
datum conversion.  Are you confident of the towgs84 value you
selected?  There are "handedness" issues between common european
and american towgs84 parameter listings for instance.  You might also
want to try with towgs84=0,0,0 and see if that is closer, or futher
away, or the same.  Perhaps the datum shift isn't being applied, or
should not be applied.

Best regards,
-- 
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