Georeferencing Images and CAD Data
As we often say, the beauty of GIS is the new information that is generated when we bring together data from many sources based on a framework of location. The previous tutorial, about spatial referencing systems explains how data may be registered in a geographic referencing system and how GIS tools use this to create useful associations with other information. In this tutorial, we can see how to add georeferencing to scanned maps, aerial photos and computer aided drafting files so that their value can be multiplied through association with other data. The GSD GIS Manual Page, Sources of Geographic Images provides links to resources where you can find an infinite amount of image-based GIS data. Some of it is georeferenced already. Some scanned maps have coordinate system information in the margins that can be used to georeference the image without reference to any other data. This can be useful for beginning a spatial framework for areas, like China or the UK where good controled GIS layers are illegal or prohibitively expensive to obtain. Where good GIS layers can be found, the process of georeferencing an image is surprisingly easy.
| |
References
- Sources of Geographic Images
- Using ArcMap pages 309-312
- Georeferencing a Raster Dataset ArcMap 9.2 Help
- Transforming CAD Datasets ArcMap 9.2 Help
Tutorial Dataset
Right-Click here to open the zip archive containing the sample dataset. Create a folder for yourself, if necessary in c:\temp Expand the zip archive into to c:\temp\yourfolder
Issues to consider when georeferencing scanned maps and aerials and CAD files
- Choice of projection: The projection you choose for your view when georeferencing determines the quality of the result. If you choose a map projection that is similar to that used by your scanned map, your georeferenced map may fit nearly perfectly with the control map with only a few control points. If you choose a projection that is not similar to the scanned map, then you will find that no matter how many control points you add, even with higer-=order transforms applied, you will only be able to get the map to fit well in very limited areas.
- Choice of a control map: You can see by examining the streets layer over the aerial photo, that the streets layer does not fit the aerial very well. Examining the metadata of each dataset lends more credibility to the airphoto. We don't always have two layers to check against eachother, but you should always keep in mind that part of the difficulty you may have in georeferncing a scanned map, is that if the scanned map or the control map have spatial errors, then your fit will not be very good. SO try to find the best control layer that you can.
- Georeferencing with aerial photos: Unrectified Aerial photos are not maps. That is, their scale is not consistent. Hills, and distance from the center of the photo can cause different parts of the map to be distorted different ways. It is often impossible to transform a photo so that all areas fit a control map well. So do the best you can with the areas that are imnportant. Keep in mind that tall things like buildings that aren't directly under the camera, are displaced. So even though the top of buildings may be the easiest things to see in a photo, try to pick your control points from the ground.
- Georeferencing Orthophotos: Most modern airphotos that are produced for planimetric surveys are Ortho-Rectified, which means that they have had distortions removed. Often the displacement of buildings as discussed above can be a factor, but if you are careful to pick your control points from the ground, you can get these to fit well.
Georeferencing a Map Image
The ArcMap help document, referenced below, does a good job of explaining the techniiques of georeferncing using the georeferencing toolbar. In this exercise, we will apply this using a scanned US Geological Survey Map from 1894. This will provide a sense of how this works and some of the issues that may make it work better or worse.
References
- Hiding and Showing Toolbars
- Georeferencing a Raster Dataset ArcMap 9.2 Help
- World Files for Raster Datasets
Georeference your 1894 Map
- Open the ArcMap document Georef_tut from the docs folder of the sample dataset.
- Take a look in the USGS folder of your sample dataset. Notice that it has a simple jpeg file and some metadata. This is a scanned USGS map from 1894.
- Open the image in ArcMap.
- It does not show up where we would hope it would. Find it by Zooming to layer on it.
- Mouse over the top left corner of the image and not its arbitrary coordinate system has its origin (0,0) there.
- Look at the spatial referencing properties of this image. There are none.
- Now zoom to your fields point layers and adjust your zoom-frame so that it is similar to the frame of the image.
- Load the georeferencing toolbar (View->Toolbars->Georeferencing)
- Turn on Auto Adjust in the Georeferencing pulldown.
- Set your 1894 map as the target layer.
- Choose Fit to Display from the georeferencing pulldown.
- Add a couple of quick rough contol points to get the image in roughly the right place.
- Carefully add 3 more control points more precisely
- Examine the links table. Note that once you have more than 3 points, this table shows you which of your points fit best with the simple first order transformation.
- Delete the points with the biggest errors and notice how the fit of all of the other points improves.
- From the Links Table, save your control points as a text file in your USGS folder. This will allow you or someone else to pick up and refine this georeferencing project later.
- Take a look at the files in your USGS folder.
- Choose Update Georeferencing from the georeferencing toolbar,
- Notice that Update Georeferencing has added a World File to your image!
- Check the spatial referencing properties of your image. Note that it has inherited the CRS from your dataframe.
Georeferencing CAD files
Georeferencing a CAD dataset is very similar to georeferencing an image. See the help document referenced below to read about key differences. A couple of things that aren't discussed include the fact that you are only allowed two control points for a CAD file links table. Also, you have to use ArcCatalog to update the Spatial Referencing settings of the CAD file when you are done.
- About CAD World Files
- http://webhelp.esri.com/arcgisdesktop/9.2/index.cfm?id=2581&pid=2578&topicname=Transforming_CAD_datasets"> Transforming CAD Datasets ArcMap 9.2 Help
- Practice georeferencing an image ad discussed above to learn the basics.
- Try georeferencing the Fields Point Base Map.dwg from the sample dataset.
- Observe the world file being created.
- Go to ArcCatalog and update the spatial reference properties of your CAD file to match those of your dataframe.
Bootstrapping a GIS: georeferencing without a control layer
It occaisionally happens that we want to build a GIS for an area for which our best coordinate system control is on a scanned map or aerial. In this case, if the image has places marked with known coordinates, we can use the Link Table feature of the georeferenfing toolbar to manually enter the coordinates for the control points (after creating a row in the table by clicking on the image point, and completing te link by clicking anywhere else. THis process is explained at the bottom of the ArcMap Help document on Georeferencing a Raster Dataset.
This image from Google earth shows how placemarks were created, and labeled with their latitudes and longitudes (after having set the Tools->Options to show coordinates in degrees. We then saved an image that includes our tics. It is important that the dataframe in arcmap be "projecting" the image in Geographic Coordinate System with reference to the Geodetic Spheroid of 1984 -- Unprojected GCS, WGS84 Now, we trick the georeferencing tool by clicking on a control point on the image, and then clicking again. This enters a link in the links table with the correct image coordinate. We can then hand-enter the map coordinates.
SOmetimes we have a scanned map with mysterious coordinate references along the edge. These sample images come from a scanned map of Shanghai. Some experiments with rough GIS data from the ESRI Data collection shows us that these are likely to be coorinates in the UTM system, Zone 51. The coordinate tics along the margin that specify kilometers in the Universal Transverse Mercator zone 51. If you look at the bottom left corner, you will see the full coordinates in meters. Find intersections of the grid lines near the four corners of the map, and identify their x and y coordinates in meters (not kilometers). Jot these down on a piece of paper. The numbers running up the side of the map omit several digits for the sake of clarity. Only at the bottom rigth corner do they show you the whole numbers: See picture 1 and Picture 2.
To georefernce this image, you would begin by projecting your dataframe to UTM Zone 51 (probably using a Chinese local Datum) and then hack your links table as instructed in the ESRI Help Document. YOu now have a reference framework for detailed information you may collect in Shanghai with your GPS!