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.
"The true functionalism of architecture must be reflected mainly in its functionality from a human point of view. Technical functionalism can not define architecture."
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.
What it means to georeference.
To georeference, at least four aspects must be considered:
1 Adjust the units to meters. 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:
X = 3,273,358.77
Y = 4,691,471.10
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.
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.
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."
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: