7 replies to this topic

[J. Harris]

Hi all.
My first post here, I'ts 20:00 and I've been sitting here trying to get this working for 3 hours. I'm going home

Here's what I have.
I was given a dwg CAD data set with no projection. I am trying to make it line up in an existing ArcMap with data from the same area. I have tried assigning it many projected coordinate systems and it always projects many miles to thousands of miles off. I went through the process on ESRI's web site here HowTo: Create custom projection file in ArcMap to align CAD data and got the data to line up ontop of itself however, the cad file draws much to large. It needs to be scaled down. see the first attachment "cad projection". The orange polygon is my GPS data, you can see where it needs to line up to on the CAD file.
There is also a screenshot of the Layer properyies "Layer props", showing the current extent.

Any help, directions to a step by step tutorial or links to another thread would be greatly appreciated.
thanks!

Attached Files

•  CAD_projection.jpg   46.21K   35 downloads
•  Layer_Props.jpg   43.69K   29 downloads

[s hubbard]

Hi all.
My first post here, I'ts 20:00 and I've been sitting here trying to get this working for 3 hours. I'm going home

Here's what I have.
I was given a dwg CAD data set with no projection. I am trying to make it line up in an existing ArcMap with data from the same area. I have tried assigning it many projected coordinate systems and it always projects many miles to thousands of miles off. I went through the process on ESRI's web site here HowTo: Create custom projection file in ArcMap to align CAD data and got the data to line up ontop of itself however, the cad file draws much to large. It needs to be scaled down. see the first attachment "cad projection". The orange polygon is my GPS data, you can see where it needs to line up to on the CAD file.
There is also a screenshot of the Layer properyies "Layer props", showing the current extent.

Any help, directions to a step by step tutorial or links to another thread would be greatly appreciated.
thanks!

i think you need to georeference the CAD data, not try to project it.
CAD data takes a "2-point similarity transformation" with georeferencing.
i've never georeferenced against GPS data, but I think it should work fine if it's already loaded into ArcMap, with spatial reference.....
start here: ESRI - Transforming CAD Data Sets

[J. Harris]

Thank you Mr. Hubbard. I think i'm 90% there. It looks like I need to learn how to use ESRI's help pages better. Thanks for directing me there. I was able to georefrence the cad data to a close representation by using the 2 control points. I chose the southeast and southwest corner of the site. The east west accuracy seems very close but the north south is several feet off of my base files. Is this something I can remedy using a different projection or is there another method. As it sits I can work with the data but I would like it to align more accurately. Thanks. oh and below is an updated screenshot. the map projection I'm currently using is NAD 83 Zone 12N. My file is the dark red bordered polygon.

My first thoughts when looking at it were just construction on the ground not matching up with the plan. Just wanted to make sure there was nothing else it could be.

Attached Files

•  GeoCAD.jpg   213.04K   33 downloads

[Melita Kennedy]

Do the layer extents you posted accurately reflect the feature's extent? Or is it picking up values based on other information in the CAD file? The max values look like they could be State Plane zone values. A few things to think about--

1. Does the CAD file look like it's using feet or meters?
2. What SPCS zones are used in the area? Or possibly a statewide system.

If you tell us approximately where the data should be (city, or county at least) and we can check that possibility.

The CAD georeferencing uses a Helmert/similarity/conformal transformation. It translates, rotates, and has a single scale (scales the same in both x and y). It's possible your data just doesn't fit that.

Regards,
Melita

[s hubbard]

you can always delete the control points you have made, and try new ones to see if you can get the RMS error down some more. try to use control points on multiple intersecting lines within the CAD and GIS data sets. i like to start the first control point near the center somewhere, then do the second towards the outer reaches. you should be able to get it closer than that...
see how both your control points are near the bottom, try separating them, one at top, one at bottom, or one in center somewhere, then the other one on the outer dark border.

[J. Harris]

Thanks for your help guys.

By changing projections (State Plane, Colorado Central (Feet)) and control points I did get some overlay improvement. In talking with some of the construction personnel, It looks like there were instances where construction on the ground did not match up with the plats. I was able to make the data work for me by clipping out some of the unneeded features. Thanks again to both of you. oh. and Mr. Hubbard, I looked at your website and was impressed. Mapping and analysis for the public is definitely where I would like to be someday.

Edited by J. Harris, 12 February 2010 - 01:08 PM.

[s hubbard]

Thanks for your help guys.

By changing projections (State Plane, Colorado Central (Feet)) and control points I did get some overlay improvement. In talking with some of the construction personnel, It looks like there were instances where construction on the ground did not match up with the plats. I was able to make the data work for me by clipping out some of the unneeded features. Thanks again to both of you. oh. and Mr. Hubbard, I looked at your website and was impressed. Mapping and analysis for the public is definitely where I would like to be someday.

hey J. >>
mapping is decent around here.
everybody is into the droid phones, map apps (i hate saying that), and anything good looking map-wise.
i seemed to have tapped into the local market somewhat. i recommend anyone in need of work, join the local chamber, start networking big time and you'll find it.
business are realizing now that GIS can isolate target audiences for them.
the mapping/advertising i think is keeping alot of cartographers afloat these days...oh well.

i meant to ask about the map too >> did you get it close enough?

[J. Harris]

i meant to ask about the map too >> did you get it close enough?

Yes It was close enough for what I needed at the time. The client was very happy. Thanks. I look forward to working with future CAD data sets now that i have a (very slight) grasp on it.