RTS - error in scaling Z from imperial to metric?

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RTS - error in scaling Z from imperial to metric?

Postby cbunker » Tue Dec 18, 2007 7:22 am

We recently noticed in a job that, in the process of scaling a set of points in imperial to metric (using RTS), that the Z-coordinates of the points were not scaled correctly. I've attached a portion of the log file showing the RTS details and the information on two of the points. We performed a rotation of the points at the same time, but that shouldn't have any effect, should it?

This was done using MSCAD 2004 SP3.

Pts to process : 1..251
Basept case : PtNum 13
Using base point : N=968.044488, E=1006.215766, Z=101.412000
Rotation case : 4PtNums 13..44,1007..1006
Using rotation : 0.0125 cw
Shift case : NoShift
Using shift : N=0.000000, E=0.000000, Z=0.000000
Scale case : Values NE=.3048 Z=.3048
Using scale factors : NE=0.304800, Z=0.304800
pt 5, start = (N=985.539097, E=1027.356763, Z=0.000000)
rotated 359.5835 ddd.mmss ccw to (N=985.530431, E=1027.363932, Z=0.000000)
shifted by dN=0.000000, dE=0.000000, dZ=0.000000 to (N=985.530431, E=1027.363932, Z=0.000000)
scale adjustments are based on rotated and shifted positions of points
scalexy=0.304800 scalez=0.304800 to (N=973.374204, E=1012.661727, Z=70.501622)

pt 251, start = (N=815.078423, E=1021.937050, Z=99.964000)
rotated 359.5835 ddd.mmss ccw to (N=815.071992, E=1021.874354, Z=99.964000)
shifted by dN=0.000000, dE=0.000000, dZ=0.000000 to (N=815.071992, E=1021.874354, Z=99.964000)
scale adjustments are based on rotated and shifted positions of points
scalexy=0.304800 scalez=0.304800 to (N=921.418472, E=1010.988503, Z=100.970650)


The resulting Z-values don't make sense. This is the first time that we've noticed this happening, has anyone else run into this?

Thanks,
Chris
Chris Bunker, BMath, O.L.S., C.L.S.
T. A. Bunker Surveying Ltd.
150 John St. N., P.O. Box 1180
Gravenhurst, ON P1P 1V4
cbunker
 
Posts: 50
Joined: Sun Nov 02, 2003 7:22 pm

Postby Glen Cameron » Tue Dec 18, 2007 8:16 am

Scaling is in relation to your base point. In this case point 13 which has an elevation of 101.412. All scaling happens about the base point in full 3D.

To convert your points from feet to meters in elevation, you need to scale about a point that has zero for an elevation.

If you modify point 13 so Z=0 then do the scaling as you did here, the elevation on point 251 would then be 30.469, as expected.
Glen W. Cameron, C.E.T.
City of North Bay, Ontario
Glen Cameron
 
Posts: 1395
Joined: Fri Nov 08, 2002 12:18 pm
Location: Corbeil, Ontario, Canada

Postby cbunker » Tue Dec 18, 2007 8:27 am

Of course, it all makes sense now :-). We usually only scale from imperial to metric if we've COGO'd in a plan in imperial, and Z values don't come into play then.

Thanks,
Chris
Chris Bunker, BMath, O.L.S., C.L.S.
T. A. Bunker Surveying Ltd.
150 John St. N., P.O. Box 1180
Gravenhurst, ON P1P 1V4
cbunker
 
Posts: 50
Joined: Sun Nov 02, 2003 7:22 pm


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