Georeferencing a CAD file

Although it is a subject for many basic, it often appears in the distribution lists and Google queries. No wonder, computer-aided design takes a long time under a focus of Engineering, Architecture and Construction while the geospatial issue has had a relationship more oriented to the management of the territory. We can not forget that every day Both disciplines converge, to the extent that both AutoCAD and Microstation included georeferencing as a common feature from recent versions (AutoCAD 2009 up AutoCAD 2012 y Microstation XM Up to V8i).

Although a dwg or an Dgn can have georeferencing indicated in their programs, to be opened by a GIS application that is not the same manufacturer assumes that the file does not have a geographical location. In this the georeferencing of CAD files is still a utility within programs of the same brand, for now. Neither the GIS program will do wonders if the file is in strange units or in a wrong Cartesian location.

Let's see when, why and as with AutoCAD although it is the same with any other CAD program.

Why georeferencing in CAD has its complexity

For practical purposes, the plans of buildings we do without thinking about the georeference, and this has different reasons:

  • We make plans looking for the alignment with the screen. Although in the real world the building is rotated with respect to the geographical north, at the moment of drawing little we are interested in that, we prefer to rotate the symbol of the north in the plane.
  • Generally the plans are made for constructive purposes, so we are looking for ways to facilitate the creation of cuts and facades, as well as adapting to the creation of layouts Consistent with the geometry.
  • While there are ways to create particular reference systems, it is not practical to work with an unreasonable orientation, at least when drawing the plans in traditional format and more when almost everything is designed orthogonal.
  • When we need to do a placement plan, we usually call it an image, Orthophoto Or cadastral map, we rotate and climb for the purpose of detail but rarely consider working at once in that space.

Georeferencing a map

"The true functionalism of architecture must be reflected mainly in its functionality from a human point of view. Technical functionalism can not define architecture."

Alvar Aalto

Designer of the building I'm using by example

Why geo-referencing is a necessity

The classic way of doing the planes has changed, the introduction of modeling gradually makes the programs adapt functionalities in which the 3D object is worked and the cuts or facades are a consequence of this.

While this is a trend, in most situations it continues to be made flat from an 2D plant. But we must not forget that this is irreversible, the need to make animations the analysis of spatial variables and the BIM approach increasingly hammered in the CAD applications, As seen in Revit or ArchiCAD.

Georeferencing a map

What it means to georeference.

To georeference, at least four aspects must be considered:

1 Adjust the units to meters. Georeferencing a map If we are going to send to a system projected as UTM, it will be necessary that the units are meters. In many cases, the planes can be drawn taking as units the millimeters or even inches of the English system.

This is done with the command Units. And there we changed the display of Architectural type to Decimal and in units of inches to meters. When making the change we noticed in the status bar how the form of visualization changes, however with this we have not changed the scale of the drawing, and if we measure a door that should measure 2.30 it appears as 92 that represent the inches that the 7 represents '- 7. "

So you have to scale the drawing one factor, in this case the equivalent of converting inches to meters, would be 0.0254.

  • The command is executed stairs, A reference point is chosen, the scaling factor is written and then enter.

2 Move the file to a UTM coordinate.

This requires known geographic coordinates, either can be obtained with a GPS, from a georeferenced orthophoto, a cadastral map that has drawn the building or Google Earth ultimately with the risks involved in its accuracy. In this case, for example purposes I am using Google Earth:

Georeferencing a map

1 point

X = 3,273,358.77

Y = 4,691,471.10

2 point

X = 3,274,451.59

Y = 4,691,510.47

These points are drawn with the command point.

  • The command is typed point, Is enter, The coordinate is written in the 3273358.77,4691471.10 form and then it is done enter.

Similarly for the other point. Then all the drawing that we will move is selected:

  • Command move, we click on the starting point that corresponds to the corner of the drawing without moving, and then we write the 1 coordinate; to not write it again we use the cursor arrow upwards and recover what we already typed in the previous step.

To do enter, the drawing will move us to the area of ​​interest as shown in the image. We will have to use the command Zoom extent to see. Or from the keyboard Z, enter, and enter.

Georeferencing a map

If we do not look at the points well, you can change the format using the command Ddptype.

3 Rotate the drawing

Now what is missing is to make the rotation of the drawing from the left node that we know is fine.

  • It selects everything to be rotated, command rotate, The rotation axis is marked by clicking on the left point (magenta dot), the option is chosen reference, Click on the two points that define the rotation vector, first in the magenta point and then in the red point.

Georeferencing a map

This action is similar to using the rotate command with three Microstation points, although here the horizontal is taken as the base.

What else involves geo-referencing?

With this, the file is not georeferenced. What we have done is to place it in a projected coordinate system, in which its north coincides with the geographical north and the location with the UTM coordinates.

Always when calling it from a GIS application, the system will ask the same data that involves projection and Datum. If we decide to do this With a GIS program Through a routine of rectification we must remember that it will only do so on the model, layout Will be lost and xml properties when re-exported to dxf.

AutoCAD brings a tool called Geographiclocation, that we will see another day, as well as the option of georeference and reproject of Microstation.

4 External references

Doing this process could only be for the temporary purpose of making an 3D animation, which would be enough to send the construction plant. If we want to do it as a definitive action towards existing complete projects, we must consider the external references -That we use to save ourselves working with very large files or integrating different users- but that implies the separation of matching files in a workspace. If we do this with a file, those should also be fixed.

Also the fact that sometimes the same file is copied inside the same model, For printing purposes ... while still thinking about the loss of layouts.

"Someday anywhere, anywhere you will inevitably find yourself, and that, only that, may be the happiest or the bitterest of your hours."

Pablo Neruda

I know, part of the post is a can, the end is nothing short of desperate; But sooner or later it is necessary to do it, more if a client appears to us that wants to see something like this:

Georeferencing a map

19 Replies to "Georeferencing a CAD File"

  1. The article is very good.
    It's a problem and we all try to solve it.
    Many times without success.

  2. hello my question is I mostly work with civilian 3d geolocation, and I have 600 coordinate points, since I import the data to the civil 3d and I want to georeference it, the place then the satellite image in civil 3d is fuzzy there will be a command to be able to have more visibility of the place.

  3. Well I would not know to answer you, it would have to see if you are entering correctly the points of origin or destination.

  4. Hello, I did the routine and it does not come out, it is assumed that when moving the drawing to the place where you need (with the coordinates), when moving to the new place that point should already have the correct coordinates, and mine does not give me Some coordinates and elevations that have nothing to do with the point I need, how do I do it?

  5. Well, the first thing is to enter your georeferenced coordinates into a CAD file.
    Then call the CAD file as a reference, and move it and rotate it according to the identified coordinates.

  6. Hey.

    I am writing to ask for help on this article. I really condes a lot with an exercise that I try to do but I do not get it, or I did not understand all the steps.

    1. I have a file (cad) with a flat without georeferences.
    2. I have an Excel file with 50 UTM taken directly in the plane terrain.
    3. The objective is to georeference the plane and to be able to enter the UTM as points.

    These points will then serve to continue working on the drawing, which will already be geo-referenced, but this is irrelevant, for consultation.

    Thanks for a website so complete for those we like cad, and say that if someone can help me, be saying that I have to do or with an article so that I can do all the steps described.

    Thank you very much

  7. the only thing I did not understand was how you got the points in X and Y, since in my google earth it gives me some coordinates equal to these 25 ° 43'29.97 "N - 100 ° 22'39.55" Or, or if you could tell me how change them to points in X and Y, thanks ...

  8. Thanks Mario.
    What you mention is a reality. If there were a standard that could be done by manufacturers ... although it sure is not easy.

  9. CAD files should have a "universal" file, that is, that georeferencing is recognized regardless of which manufacturer it is. I usually have to georeference DWG files of topographic surveys with an orthophoto, which many times the ArcGis does not recognize me said georeference. Congratulations on the article, I found it very practical. Regards.

  10. Actually, I have not seen many iPad applications that allow you to open a kmz file. What happens is that the kmz is a compressed file (like a .zip or a .rar), which contains within one or several kml and georeferenced images or contained within the same kmz file.

    Try to see how these work: KMZ Loader, My Maps Editor, MapBox, POI Viewer, Map Editor, GPS-Trk

    The best is GIS Pro, but it's worth a lot of money.

    Another problem is that although some of these applications claim to support composite kmz, some run having Google Earth installed for iPad, and this is not available for all countries, especially in Latin America.

  11. I congratulate you on your blog, I wanted to ask you a favor, if you could comment on how you can upload a map that was CAD and I converted it into a polygon (photo) to a KMZ file and I can not see it on my Ipad. I've tried everything and I can not. It seems very obvious, but I do not know how to do it. There are some applications that allow loading KMZ, but very limited (only positions), Thank you very much!

Leave an answer

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.