Georeferencing Tutorial
Georeferencing allows you to take a scanned map, air photo, or satellite image and give it a spatial reference on the Earth, so you can use it in a GIS for spatial analysis and for creating additional GIS data. For our example we will georeference the Spartanburg Water Districts map. To properly position the map we are going to use the intersection of counties as reference points by linking the water districts map to the Counties_SC shapefile.
1. First, copy and paste the entire LadnerCI folder from the libstorage drive to the hard drive of your local machine. For example, I have copied and pasted the folder to my D drive, so the path to the folder is D:\LadnerCI.
2. Open a new, blank map in ArcMap. Use the Connect to Folder button to connect to the LadnerCI folder on your hard drive (e.g. D:\LadnerCI).
Set the Coordinate System for your Data Frame
3. Change the coordinate system of your data frame by right clicking on Layers in your table of contents and select Properties. Choose the Coordinate System tab. Scroll up until you see Projected Coordinate Systems. Select this option and navigate to State Plane > NAD 1983 (2011) (Meters) > NAD 1983 (2011) StatePlane South Carolina FIPS 3900 (Meters). Click OK. (A warning message may appear that asks you to change the coordinate system of your data layers. Click Yes if it does).
4. Add the image to be georeferenced (SpburgWater.tif) by dragging it from the ArcCatalog window on the side of the screen. ArcMap immediately warns you that your data as no spatial reference - because we haven't defined it yet. Click OK.
5. Drag the Counties_SC.shp data over to the map.
6. It may help to change the symbology of Counties_SC when we do the georeferencing. Right click the Counties_SC layer in the Table of Contents, select Properties, and select the Symbology tab. Click the symbol and change it to hollow, the line width to 1 point, and the color to a bright, contrasting color such as red.
The Georeferencing Toolbar
7.To display the Georeferencing toolbar, click the Customize menu, point to Toolbars, and then click Georeferencing.
At this point, your display should look similar to this (click the picture below to see at maximum size). Now we want to link the map with the Counties_SC dataset locations.
8. On the Georeferencing toolbar, make sure the Layer selection drop down is set to your image (SpburgWater.tif).
9. To add a link, click a known location on the raster dataset (SpburgWater.tif), followed by the same location on the Counties_SC dataset. To start, I'm going to use the corner of Landrum and Greenville counties in the northwest corner of the map. Zoom in with the zoom tool until you can distinctly see the corner where the counties intersect.
10. Click the Add Control Points button on the Georeferencing toolbar and then click on the corner of the counties. A cross icon shows up with the number 1.
11.Then, right click on Counties_SC in the table of contents. Use the zoom tool to zoom in to the same location. Click the Add Control Points button again to activate the tool, and click on the county boundaries to add the link location.
Tip: Press ESC to remove a link while you're in the middle of creating it.
12. Repeat this process using 3 additional points on the map: the southernmost point of Union County, the eastern edge of Cherokee County, and the intersection of Greenville and Landrum Counties in the southwest.
After adding these four points, the county boundaries line up nicely with the map.
Saving your results
We will permanently transform our new georeferenced raster dataset by using the Rectify command to export a new .tif file.
13. On the Georeferencing toolbar, click Georeferencing and click Rectify. Choose bilinear interpolation as resample type. (This interpolation method results in a smoother-looking surface than can be obtained using nearest neighbor (the default)). Save the output as SpburgWater_georef.tif in the LadnerCI folder (e.g. D:\LadnerCI).
Advanced Georeferencing Information:
We used four link points to georeference the Spartanburg Water Districts map. This is enough links for the type of transformation that we need to create our desired polygons later on. You can assess the quality of the links and the transformation by clicking the View Link Table button on the Georeferencing toolbar. You can delete an unwanted link from the Link Table dialog box by clicking on the link and hitting the Delete button. If possible, you should spread out the links over the entire raster dataset rather than concentrating them in one area. Typically, having at least one link near each corner of the raster dataset and a few throughout the interior produces the best results. As a general rule, you need a minimum of 4 links for a spline or first-order polynomial (maps), 9 links for a second-order polynomial (aerial photos), and 10 links for a third-order polynomial.Adding more links will not necessarily yield a better registration.
Transformations:
Use a first order, or affine transformation, to shift, scale, and rotate a raster dataset. This is most common used when georeferencing maps.
If, however, the raster dataset must be bent or curved, use a second- or third-order transformation. You can examine the residual error for each link and the RMS error. If you're satisfied with the registration, you can stop entering links.
Interpreting the root mean square error (RMS)
When the general formula is derived and applied to the control point, a measure of the error—the residual error—is returned. The error is the difference between where the from point ended up as opposed to the actual location that was specified—the to point position.
The total error is computed by taking the root mean square (RMS) sum of all the residuals to compute the RMS error. This value describes how consistent the transformation is between the different control points (links). When the error is particularly large, you may want to remove and add control points to adjust the error.
Although the RMS error is a good assessment of the accuracy of the transformation, don’t confuse a low RMS error with an accurate registration. For example, the transformation may still contain significant errors due to a poorly entered control point. The more control points of equal quality used, the more accurately the polynomial can convert the input data to output coordinates. Typically, the adjust and spline transformations give an RMS of near zero or zero; however, this does not mean that the image will be perfectly georeferenced.
Acceptable RMS error
Some literature suggests that it should be "less than or equal to 1/2 of the side of a cell which make up the total resolution of the image." However, there is no absolute value for RMS, because it depends on the quality of the map being georeferenced, the quality of the target (base) map, and the purpose of the georeferencing.