Opened 13 years ago

Last modified 6 years ago

#247 new defect

m.proj: mind that if input points are XYZ rather than XY, cs2cs will include Z in transfomartion

Reported by: msieczka Owned by: grass-dev@…
Priority: normal Milestone: 6.4.6
Component: Projections/Datums Version: svn-develbranch6
Keywords: m.proj Cc:
CPU: All Platform: All

Description

Take for example such 3 points (Z is elevation a.s.l. in meters):

16 52 64
16.00083333 52 63
16.00166667 52 62

The source CRS is:

+proj=longlat +ellps=krass +towgs84=33.4,-146.6,-76.3,-0.359,-0.053,0.844,-0.84

and target CRS:

+proj=tmerc +lat_0=0 +lon_0=19 +k=0.9993 +x_0=500000 +y_0=-5300000 +ellps=GRS80 +units=m

As the datums differ, cs2cs (m.proj) result will differ depending if Z is included or not in the input, because cs2cs takes the Z coordinate for ellipsoidal height. Since my Z coordinate is a.s.l., not ellipsoidal, this will introduce an error.

cs2cs result when Z is there in the input:

568525.00043548	5761469.34143594 103.09038265
568582.20740863	5761470.12874655 102.08903865
568639.41506476	5761470.91672242 101.08769464

And without Z:

568524.99893752	5761469.34103610 39.09043643
568582.20593408	5761470.12835296 39.08909159
568639.41361362	5761470.91633507 39.08774674

There is a slight difference in XY, which one might argue is neglectable, yet the bogus Z transformation might get the user into real trouble if he still treats the data as 3D after transformation.

There was a similar issue in v.proj with 3D data, and it was solved by adding a switch, but this can't be done for m.proj because there is no way in cs2cs to prevent it from processing the Z coord in input.

m.proj should propably refuse to process data with more than 2 columns.

Change History (3)

comment:1 Changed 9 years ago by neteler

Milestone: 6.4.06.4.4

comment:2 Changed 9 years ago by hamish

Component: VectorProjections/Datums
Keywords: m.proj added
Priority: majornormal

comment:3 Changed 6 years ago by neteler

Milestone: 6.4.46.4.6
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