Jump to content
TheArchaeologist

Geodetic transformation paths - how to assign them?

Recommended Posts

TheArchaeologist

Although I've been using CAD for a while now I'm a little confused as to how you assign a geodetic transformation path. It's fairly straightforward to apply a transformation to a coordinate system in ArcGIS, but CAD Map seems determined to complicate matters for me!

 

I have maps that were drawn using the ED50 projection and I want to align them with satellite imagery in the UTM84 (WGS84 zone 36N) coordinate system. There is already a transformation path available (ED50-IGN.ES_to_WGS84), but when I view it in the library I cannot assign it as the assign button is greyed out. I have found the CAD help files to be less than helpful, so I was wondering if anyone could point me in the right direction to get this working?

 

My coordinate system for the drawing is set to UTM84-36N, I have also tried it with the coordinate system set to ED50-UTM36, but thus far no joy.

Share this post


Link to post
Share on other sites
Murph_map

How are you bringing the images into the Map3D? What format are they in? Where is this transformation path (ED50-IGN.ES_to_WGS84) located at and what is it?

Share this post


Link to post
Share on other sites
TheArchaeologist

The maps that I am using to digitise contours, structures etc. are hi-res jpeg files that have been attached to my file using IMAGEATTACH, then georeferenced to the correct coordinates based on the graticules shown on the maps. Each map was then rubber sheeted to remove distortion. The transformation path can be found in the Coordinate System library using the Geodetic Transformation Path filter and typing ED50 into the search. It appears to provide a transformation from ED50-IGN.ES to ETRF89 and from ETRF89 to WGS84.

 

I am using CAD map 2015 and I cannot find a way to apply the transformation to the coordinate system. I have been through the process of creating a Geodetic transformation path from an existing path (which seems like an odd thing to do, if there is an existing path definition then why can this not just be applied to the current coordinate system?) but nothing happens.

Share this post


Link to post
Share on other sites
Murph_map

When you are rubbersheeting the images are you creating your own coordinate system? Why not just insert/attach the images in the dwg with the correct coordinate system to start if you are having to rubbersheet them anyways? Now if the images are already in a know coordinate system and the geo-reference files/info are included with the images use the MAPIINSERT command or data connect to bring them into the maps.

Share this post


Link to post
Share on other sites
TheArchaeologist

No, I'm rubber sheeting the images to the assigned coordinate system. They require rubber sheeting because they are scans of maps from the late 1970's that were printed onto paper and so have suffered from distortion over time. Sadly there are no georeference files for these maps.

 

The issue is that I have maps from the 1970's based on an ED50 projection, and field survey data from 3 years ago in the WGS84 coordinate system which I need to combine. Without a geodetic transformation the two sets of data are offset from each other.

Share this post


Link to post
Share on other sites
Murph_map

So are you saying that if you assign the ED50/ES.LL coordinate system to your dwg that contains the old maps and then mapattach and query in the WGS84 dwgs they do not reproject correctly?

Share this post


Link to post
Share on other sites
TheArchaeologist
So are you saying that if you assign the ED50/ES.LL coordinate system to your dwg that contains the old maps and then mapattach and query in the WGS84 dwgs they do not reproject correctly?

 

Yes, although I've not been querying the data in. Perhaps this may be my problem? I have the WGS84 field data as SHP files from ArcGIS. I do a data connect in CAD Map to bring the SHP files in (to the drawing using the ED50 CRS), it recognises their CRS but does not give an option to perform a transformation when you add to map and they do not project to the correct location. I've just looked at the 'Add to map with query' option but I cannot see anything on there for transformations. This is an area of CAD that I'm not very knowledgeable in, having done very little with map projections previously. Any guidance in the right direction would be most appreciated.

Share this post


Link to post
Share on other sites
Murph_map

The query that I mention is when you use the "ADEDRAWINGS" command. If you are using the Data Connect make sure you see both coordinate systems when you connect. See attached.

 

coordinates.png

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×