Sunday, July 25, 2010

CAD to GIS Conversion - An overview with a solution

Since times immemorial CAD drawings have been used in all kinds of Civil Engineering works. CAD offered lot of advantages to a user - to work on large amount of data in less time, easy storage and retrieval, more accuracy, easy editing, options to duplicate and so on. Only thing CAD drawings lacked is intelligence. By this I mean, each CAD entity is not connected to another one at all. If there is one highway and another village road, both are placed in different layers and their intersection can be seen but cannot be used for analysis like traffic flow. As they aren't connected if highway is moved other road stays where it is. On the other hand GIS which came in at a later stage did not have these disadvantages. GIS too organized data in layers, but layers possessed intelligence. There was connectivity between elements and analysis could be easily carried out. To add more, GIS came along with facility to define the geographic location(Nowadays, even AutoCAD has this facility) which meant that the data could be overlaid on any world map!

All these tempted the CAD users to migrate their data onto GIS. GIS offers to store data in various formats like databases, while CAD is largely file-based. CAD drawings are usually tiled across various files. Sometimes it so happens that, these tiled drawings use different conventions and scales! Adding to the woes of migration are data arrangement in layers which are mixed up badly. It so happens that text components are placed in road layer which is actually supposed to have polylines only. Recent blog article on CAD and GIS (Integration and beyond) highlights the ways of making CAD data GIS ready with AutoCAD Map 3D.

CAD - GIS Conversion using ArcGIS:
ArcGIS, a leading GIS software product, has direct read capability to visualize CAD drawings. This feature enables a user to directly open an AutoCAD drawing on ArcMap. As it is rightly said on ArcGIS Desktop Help, the CAD drawing so added will have plenty of layers in the form of polylines, polygons, text and points. These layers visualized are not the same as what is drawn in AutoCAD. For example, a CAD drawing will have layers like 0, roads, buildings, culverts, bridges, electric pole locations etc. But what ArcGIS does is it extracts point, line, text etc. from each of the layers separately and clubs them (Fig. 1). Fig. 2 shows a limited number of layers out of large number of layers which are extracted. One can imagine if points have such a large number of layers, what will be the number of layers when all point, line, polygon... are combined together. The user can visualize the data exactly as in AutoCAD drawing but he has to import the features without knowing to which layer they belong to. This gives less choice to the user in terms of weeding out unnecessary features. A screenshot of how layers are seen in ArcGIS is as below:
Fig. 1 Arrangement of layers in Table of Contents - ArcMap
Fig. 2 Point layer explained
Again the georeferencing suggested for CAD drawings is simply funny. Facility is provided to use only two-point transformation. This method is bound to work only for a small piece of drawing which has preferably well distributed four corners. If one has a linear feature like a long stretch of a road or a pipeline it becomes a herculean task and the outcome will not be encouraging. This is exactly the situation I faced and am sure that many others would have faced it.

How much ever you google for CAD to GIS conversion procedures, the above one is the only procedure widely publicized.

But there is definitely an alternative and it is the best solution for such problems. It is to use FME Desktop or Data Interoperability Extension of ArcGIS which is a paid one and not available even with ArcInfo license. More about this software/ extension and how I used it to convert AutoCAD DWG files into ESRI Shape in my next post.