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{LaB} » Georectifying / Georeferencing
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Georectifying / Georeferencing

Georectifying is a tool that allows you to transform paper maps, CAD files or other images into an ArcGIS layer. This is handy especially for historic maps, aerial photos, data found on-line that is not digitally available, or print data. This can also be of use when you do not have the information you need to georeference the material – such as no data regarding source datum or when you have a CAD file that naturally has relative coordinates. What this does is to allow you to manually match up raster data with vector data or in the case of CAD files, vector and vector.

  1. Open ArcMap, either using a previous map that you would like to use to add data to, or create a new one and add reference layer (used to fit print object to).
  2. Add image file to map (Add data button). In dialogue box asking if you want to build pyramids, choose yes. Don’t worry about the error message saying it lacks projection, because this is what you are doing.
  3. View > Toolbars > Georeferencing
  4. In the layer drop down menu, make sure the image file is listed.
  5. Within the toolbar are several tools to allow you to fit the image to a shapefile (or whatever you want to use). In the drop-down menu, fit to display will transpose the image on top of the shapefile. If you need to, rearrange the order of the files or make layers transparent so you can see.
  • a. Rotate tool rotates the image, Move Tool (under Rotate) allows you to move the image independently of the shapefile.
  • b. Add Control Points tool allows you to stretch and modify the shape of the image to distort to the shapefile. Control points act like anchors and allow you to find readily identifiable landmarks on both maps. Add control points until the image is closely aligned. It is usually not possible to match 100% so focus on the most important portion.
  • c. Delete control point icon
  • d. When satisfied click rectify. This saves the image as a new file with spatial information attached to it that you can add to the map.

When you georectify CAD datasets, if you have multiple feature layers from the same dataset, you can use any layer for the transformation and it will apply to the entire dataset.
You can enter specific coordinates for control points by using the Add Control Point tool > Click the from point > RtClick, select Input x and y > enter exact coordinates > OK
Once a transformation has been made to the CAD dataset, you can store the transformation in a world file. (Update Georeferencing >) Click Link Table Save button > Save As > browse to folder, name transformation > OK (to associate a world file with a CAD dataset, you must use the CAD dataset’s prefix name and place it in the same folder where the CAD drawing resides. You can also create a universal world file for all CAD drawings within a folder by saving the world file as esri_cad.wld – if there is a particular world file associated with a CAD file, it will override the universal.)
To load a world file: click Link Table button > click Load button > OK

Importing and Using CAD Files

CAD files can be added to a map in either native AutoCAD (*dwg) format or Drawing Exchange (*.dxf) format. They can be viewed but not edited. If you are using AutoCAD 2010 you must save your CAD file down a version to 2007.

Add a CAD layer as a layer for display

New empty map > add data > choose CAD file (CAD files will be a dataset containing multiple features; select the dataset) > Add

Add CAD file as a layer for edit and analysis

Instead of selecting the whole dataset as a layer to add, navigate to the inside of the CAD file and select a feature to add (such as polyline). The features added to the map can be selected and their properties changed, however they cannot be edited. Entities in CAD are stored as homogenous geometry types. With special software you can export specific feature classes.

Using the symbology tab in the layer properties, you can use categories = unique values, value field = layer, add all values to change the line colors and widths as desired.

Exporting a shapefile to open in CAD

ArcCatalog > RtClick on shapefile in catalog tree > copy > browse to where you would like exported file > RtClick on folder > paste > RtClick copied file > Export, Shapefile to DXF > Decimals = 6 (decimals = number of decimal places used for the coordinates in the output), Output = browse to the new shapefile, enter new name > OK



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