Monday, February 2, 2009

CAD services and Georeferencing

The word ‘georeferencing’ was originally used to describe the process of referencing a map image to a geographic location.

To describe it now, georeferencing is a process of taking an image and assigning geographic coordinates to it or to georeference something means to define its existence in the physical space by establishing a relation between raster or vector images to map projections or coordinate systems. When data from different sources need to be compiled and used in any GIS application, it is important to have a common referencing system. This is brought about by using various georeferencing techniques.

The process of georeferencing

Georeferencing usually refers to the method by which locations in raster and vector GIS files are related to real earth-surface positions.

Raster data is often taken by scanning maps or collecting aerial photographs and satellite images. Scanned map datasets usually do not have any spatial reference information. The location information delivered with aerial photography and satellite imagery might be inadequate and the data need not align correctly with some other data you possess.

In order to use some raster datasets in conjunction with other spatial data, you sometimes need to align or georeference them to a map coordinate system which can be defines using a map projection to display the curved surface of the earth on a flat surface.

Georeference a raster dataset means defining its location using map coordinates and assigning a coordinate system. This process allows it to be viewed, queried, and analyzed in comparison with other geographic data.

How can you georeference a raster dataset?

Add the raster dataset to be aligned to your projected data in ArcMap.

Add control points that connect known raster dataset positions to the map coordinate positions.

When the alignment becomes correct, save the georeferencing information (registration) for further use.

Though Geomatics users are familiar with the term ‘georeferening’, many of the CAD users are not much aware of the term and the use of georeferencing in CAD.

Why is it needed?

A raster image has no particular size as it is made up of pixels. The size of the vectorised CAD/GIS drawing is determined by the raster's pixel dimensions, without georeferencing. The image resolution (DPI) can determine this. This image size usually has no relationship with the size of the drawing that the raster represents.

A CAD file is usually drawn in a local coordinate system and depicts spatial information which can be drawn accurately without being fixed to a higher global coordinate system. However, when the drawing needs to be related to a higher level in a mapping context with data from many different sources, tools in ArcMap can easily reposition a CAD file and integrate it into that higher level without altering the CAD file.

First thing is to find out two points on the CAD file that matches with two points in the map for which we can use the Georeferencing toolbar in ArcMap.

Different tools on the toolbar can support different workflows and below listed is an easy way to describe georeferencing a CAD file.

First step is to load the CAD drawing and ensure that one of the CAD feature layers are listed in the drop down menu of the georeferencing toolbar.

Zooming to the place roughly on the map where the CAD drawing is to be related is the next step. You can use the ‘fit to display’ tool on the menu to fix the CAD file in the map frame.

ArcMap snapping helps accurate placement of the CAD file which makes precise selection of control points based on existing geometry. Use the ‘rotate’ tool if the CAD file needs to be rotated to get the drawing nearer to its final position. This enables easy picking of control points.

You can use the interactive scaling tool also for the above mentioned reason.

Now when you select the ‘update georeferencing’ option, this creates a .WLD file that will be read from now on to put your CAD file in the right place.

Finally, the coordinates are always adjusted into this position. Now you can select a coordinate system for your CAD drawing so that you can assign or project its coordinates with reference to the map or during any other geospatial operation.

Article By : RARIMA N S

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