AutoCAD Map 3D General Discussion

AutoCAD Map 3D General Discussion

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Registered: ‎09-10-2013
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problem with Nearmap image file & correct coordinate system

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09-17-2013 07:46 PM

Nearmap has provided an aerial image georeferenced in the same coord system as Google Earth (epsg:3785).  I found a Map 3D coordinate system WGS84.PseudoMercator (epsg:3857) which on web forums is equivalent to epsg:3785.  After loading the image into a dwg on the epsg:3857 coord system I then queried the image into a dwg file on an mga55 gda94 coord datum.

It rotated the image correctly but as we had survey located features in the drawing the image needed to be shifted a further 1.75 metres SW than the transformation that Map had performed.

My question is can anyone confirm that the Map epsg:3857 is indeed the same as epsg:3785?  Given I had to shift the image I suspect it isn't.

Nearmap have also suggested using an epsg:900913 coord datum but there isn't one in Map 3D.

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Re: problem with Nearmap image file & correct coordinate system

09-18-2013 12:43 AM in reply to: Greg-Linguey

You can compare the EPSG contents here: as you can see in the "Retrieve code" report, the 3785 refers only to New Zealand.

 

About EPSG:900913 it's a funny way to portray the official EPSG:3857, check this out:

 

9 => G

0 => O

0 => O

9 => G

1 => L

3 => E

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Re: problem with Nearmap image file & correct coordinate system

09-19-2013 06:06 PM in reply to: antoniovinci

Thank you for the reply.  I went to the link & it is interesting that epsg:3785 only applies to the south island of New Zealand.  From correspondence with Nearmap & the image name (starts with epsg_3785) this is the datum they are using.  I would assume that Nearmap would know what they are doing but as our imagery is based on the south-east coast of Australia it might not be the correct datum to be using.

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