[Proj] GRS80 vs GRS80 +towgs84=0,0,0

[Maciek Sieczka]

Hi,

I've encountered one small thing using cs2cs I don't understand. There
is a difference in results when converting between WGS84 and GRS80 based
systems - when the towgs84=0,0,0 is given for GRS80 and when it is not:

cs2cs -f "%.6f" +proj=latlong +ellps=WGS84 +to +proj=tmerc +lat_0=0
lon_0=19 +k=0.9993 +x_0=500000 +y_0=-5300000 +ellps=GRS80 +towgs84=0,0,0

17d01'0.0E 51d07'0.0N
361227.112181   362969.278159 0.000063

But, if I don't specify the +towgs84=0,0,0 in the target coordinate
system, I obtain a slightly different result:

cs2cs -f "%.6f" +proj=latlong +ellps=WGS84 +to +proj=tmerc +lat_0=0
+lon_0=19 +k=0.9993 +x_0=500000 +y_0=-5300000 +ellps=GRS80

17d01'0.0E 51d07'0.0N
361227.112178   362969.278057 0.000000

The difference is less than 1 mm, but anyway - why? I thought that no
matter if I specify the towgs84=0,0,0 for GRS80 or not I would obtain
the same results. 

Maciek


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