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drtees92 post(s)
#24-Apr-17 18:48

I am running Manifold 8.30.0 on Windows 7. I have in the past successfully exported GIS information to DXF format for our landscapers. They use AutoCAD 2015. Typically, there is little difficulty in the transfer of data. If the data I export does not align with survey information in our CAD files, it is almost always because the CAD file is not projected data. When this occurs, I paste a parcel outline on the data I export, which allows the landscapers to manually register my data to their files. This has worked well until now.

Recently, the data I export as DXF does not import into AutoCAD correctly. It always comes in at coordinate 0,0,0 and at 1/6th the size. I have checked the projection of my data and imported the CAD data into my Manifold map. The data are properly projected and the imported CAD data registers correctly with the GIS data.

Things get even more curious than the above example. If I create and export area contours as DXF, not only does it come into AutoCAD at 0,0,0 and 1/6th the size, but any distance measurements made on the imported data make no sense at all. However, if I create and export line contours as DXF, it will import into AutoCAD at 0,0,0, but the distance measurements make some sense (albeit 1/6th too short).

I tested this issue with Dassault Systeme's Draftsight software and replicated the same results as in AutoCAD, so I know that our current version of AutoCAD is not the issue.

I have attached a Map file, and a DWG file with topography data already imported. The DWG file contains topography created from lines and from areas. These are displayed side-by-side and have a line measured at exactly 120 units copied over each topography layer. The leftmost example is the imported line topography. Measuring distance on example line comes out at 120 units. The rightmost example is the imported area topography. Measuring distance on the example line comes out to something unusually large.

The Map file contains the original LIDAR data used to create both the line and area contours. Area contour data is modified to remove the "<" token and change the data type from text to floating point (the technique I have used for several years now).

Attachments:
Manifold Upload.zip

Dimitri


6,233 post(s)
#24-Apr-17 21:24

Sounds like a projection issue. Your data in the .map is projected, but it appears to be projected correctly. To double-check I imported it into Radian and then overlaid it on a variety of Google-served layers, and everything lines up (image below).

DXF, if I recall correctly, is not a projected format so somewhere between exporting projected data to the DXF format and then doing whatever you have to do to import it into AutoCAD there is a disconnect. I'm not an AutoCAD guy so the best I can do is to confirm that there appears to be nothing wrong in the projections in Manifold.

Attachments:
looks_ok.png

drtees92 post(s)
#24-Apr-17 22:04

Essentially the same issues that I have been encountering. AutoCAD data appear to align correctly in the map, but exporting to .dxf creates a disconnect with the contours. AutoCAD is not a mapping program, but can replicate a projected coordinate system in model space. Civil engineers will often not project their development data in AutoCAD until after finishing the site survey. Then, it is relatively easy to move their base point to the state plane coordinates for the benchmark used.

In the past, I have had success in exporting data to .dxf and have it align reasonably well in our AutoCAD files. Out of habit, I also copy parcel boundaries into the exported data to ensure that it aligned correctly, or to be moved easily into alignment with existing survey data. However, exporting contours generated from LIDAR is new to us. We have a couple of extremely large projects where getting survey-based topography is still months out. We usually rely on survey-generated topography due to the accuracy limitations of LIDAR in areas of dense vegetation and that the elevations generated by LIDAR differ from survey data by a foot or more in some areas. In my line of work, an error of a foot in elevation can spell the difference between success or failure in wetland mitigation projects.

I am attempting to use our TopCon GMS-2 GPS receiver to translate the GIS data directly to a .dwg file. It has been crunching the data since last Friday and is only half way through the process. In the meantime, I will think about other workarounds.

tjhb

9,452 post(s)
#24-Apr-17 22:27

I'll check the file but this looks like a classic example of creating a drawing with local offsets and local scales to match a related raster (image or surface), and not removing these by reprojection before export.

Yes, that's it.

There are loads of examples on the forum of others having the same problem when operating together with other packages. You are not alone--but you only need to get your head around this once.

I'll find some example links with further reading, but in the meantime:

Before exporting any drawing for use outside Manifold, always make sure that its projection has local offset values of (0, 0) and local scale values of (1, 1). Use the Assign Projection dialog to check this.

If a drawing has other values for local offsets or scales (as will usually be the case for a drawing created with, or from, any raster, or alongside a map containing a raster), then use the Change Projection dialog to change local offsets to 0, local scales to 1. This does not change the location of the data, but changes the numbers expressing location to match what is expected outside Manifold.

(Don't change anything else.)

Here are some related links.

http://www.georeference.org/forum/t118863.15

http://www.georeference.org/forum/t120952.8

http://www.georeference.org/forum/t107602.6

drtees92 post(s)
#24-Apr-17 22:32

I believe that I just discovered this for myself! Had a thought while replying to Dmitri that I should turn the telescope around, so to speak. It appears to have worked!

CuDomagoj70 post(s)
#27-Apr-17 09:19

Autodesk has little complicated in that. But it has a simple solution. I do not know if you are using Autodesk Map 3D, Civil 3D or just Autocad. If it's a Map 3D or a Civil 3D, do not select a projection (just draw with x, y coordinates.) Open the drawing where you have parcels and load the layer of the query builder (Attach drawing), then you can choose a coordinate system.

BCowper


1,272 post(s)
#27-Apr-17 14:51

Assigning a coordinate system in Civil 3D or AutoCAD Map does not resolve the local offset/scales issue. Better to sort the issue in Manifold as Tim suggested.

drtees92 post(s)
#24-Apr-17 22:30

Problem solved! I took the parcel data I knew would export correctly and copied the LIDAR topography onto it. The exported combined parcel and topo data came in where it should have and at the correct scale.

One thing I noticed in the projection information for the LIDAR data is that the local scale factor is set to 6.00, while the parcel data local scale factor is 0.00. I don't touch these numbers if everything appears to be coming together correctly. I did play with the projection scale factor on the data being exported. The results were not good. it is probably an idiosyncrasy of the .dxf file format that has trouble with the local scale factor value. Copying the topo data to a layer with a local scale factor of 0.00 appears to eliminate this problem.

steveFitz

261 post(s)
#28-Apr-17 06:47

As mentioned by Tim the issue is most likely due to your vector drawings having offset and scale. It is worth looking at Tims Links.

This often happens when you import or create a Drawing when you already have an image/raster that has offset and the Drawings adopt the offset and scale.

It has caused me so much grief in the past when I've exported to shapefile for field use only to find they don't have a working projection when out in the field with my GNSS... 100ks from the office.

I now make it a routine task to assign and verify all components in each project.

An easy way to do it is using an Add-in such as the attached (credit to all who wrote the original code that I cobbled together some time ago). When run the add-in will prompt you if a selected Drawing component has offset or scale and offer to set it to 0 and 1.

I used the add-in on the project you posted and found none of the components had their coordinate systems verified and some had incorrect offset/scale (which it fixed). The coordinates of all dxf exports created were then correct. This all took about 20 seconds!

Steve

Attachments:
CheckCoordSys.zip

drtees92 post(s)
#01-May-17 22:01

Thank you, Steve. I look forward to trying out the script.

One of the benefits of Manifold is that it quite ably projects drawings on the fly. The downside is that some older programs cannot handle drawings that aren't all projected exactly the same. For whatever reason, the LIDAR data I get locally often has a local scale factor of 6. Since Manifold doesn't have an issue with this, I seldom correct it out of fear that my correction will cause other nasty consequences. Other raster imagery I use sometimes has a local scale of 3. Projecting these to whatever Map I am using often does not change the scale factor.

Another "interesting" feature of GIS data I get from local agencies (one in particular) is that, while it uses the correct State Plane projection and units, the false easting is not correct. I suspect, but cannot confirm, that the data were entered as SAE units on a metric projection. Something to worry about in the USA since the government often works in metric, but publishes in SAE for the benefit of the rest of the public. Projecting these drawings to a Map using the correct false easting brings the data back into proper registration. Other local agencies publish their GIS data projected using UTM coordinates. Since we are closer to the Artic Circle than we are to the Equator, UTM begins to have distortion issues. Mapping would be so much simpler if the Flat Earth Society were right!

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