Image Geo-Referencing in ArcGIS Christopher M. U. Neale Utah State University For GIS in Water Resources

Image Geo-Referencing Though most image data sources now days are geo-referenced, there are many situations for which further coordinate adjustment is needed: Constant x, y, misalignment between image and vector layer due to poorly performed coordinate transformation for example Irregular misalignment (different x,y and rotational component)

Image Adjustment Image coordinate adjustment can be conducted within ArcGIS using the Geo-Referencing tool An image without geographical coordinates can be geo-referenced either to an existing vector layer or an image map such as an orthophoto, both with appropriate coordinates and projections.

Geo-referencing Using and Image map Open ArcMap with an empty view and add the image to be rectified or adjusted and the image base map using the “add data” button. Zoom to full extent. The image with no coordinates appears as a dot in this case as it doesn’t have coordinates yet as the base image on the lower right hand corner. Un-registered image Image Base Map

Geo-referencing Using and Image map In the Georeferencing drop-down menu select Fit to Display. Now both the satellite image to be geo-referenced (the larger image) and the image base map (the tiny image) are displayed together. The blue coloring is due to the way the spectral bands of the satellite image are being displayed on the RGB monitor.

Changing the Image Display Color Scheme By selecting that layer on the left hand side of the ArcMap window, right clicking on the name and selecting properties the following window will open. Select the symbology tab. The order in which the spectral bands of the satellite image are displayed on the RGB monitor is shown. In this case the near-infrared band (band 3) is on the blue color, resulting in blue colored vegetation.

Changing the Image Display Color Scheme Changing the band combination by using the drop down arrows on the right hand side, we can place the near-infrared band (band 3) of this satellite imagery on the red color and the green band (band 1) on the blue color, rendering the classic false color composite coloring. Band 2 is the red band in this case. The satellite image is a portion of an ASTER image, 15 meter pixel resolution.

Using the Geo-Referencing Toolbar First we need to make sure the Geo-referencing tool bar is active by clicking on the View Menu, click on Toolbars, and select the Georeferencing Tool Bar. Dock it wherever you choose: IMPORTANT!: Before starting, click on the Georeferencing dropdown menu and “uncheck” the Auto Adjust option.

Adding Control Points Click on the Add Control Points button (circled in red below). The cursor will turn into a “plus sign” or “crosshair” when placed over the image. Always select the control point on the image to be geo-referenced first. Once a pixel is selected the crosshair will turn green:

Adding Control Points Then select the corresponding point on the image base map. You will need to unselect the image to be geo-referenced in order to expose the image base map, allowing you to zoom in to the area of the corresponding control point. Once identified the area, the Add Control Points button is depressed again and the cursor will change to a cross-hair, and color once the point is clicked.

Adding Control Points If you click on Fill to Display again, in the Geo-referencing drop down menu, the following images will show on the ArcGIS display: The un-georeferenced satellite image on the left and the image base map on the right with a blue line connecting the respective control point pixels.

Adding Control Points A second control point is added and related to the base image. By clicking on the View Link Table icon to the right of the Add Control Point icon, a table listing the control points selected at that moment.

Adding Control Points Control points should be distributed on the image to be geo-referenced in such a way as to cover the entire extent in a grid-like fashion.

Viewing Control Point Coordinates Open by clicking the View Link Table icon to the right of the Add Control Point icon. The table lists the control points selected up to that point in the process.

Viewing Control Point Coordinates Once there are enough well distributed control points on the image to be geo-referenced, the transformation can be applied (in this case a 1st order polynomial transformation) by selecting the Update Display option in the Georeferencing Tool pull-down menu. An overall Root Mean Square (RMS) error is calculated and the residual resulting from each control point is displayed. Points with large RMS values can be deleted and re-selected.

Testing the Geo-referencing Once the display has been updated, the transformation used is applied to the image and the satellite image is geo-referenced and now overlays the image base map. The accuracy of the match can be ascertained by turning the satellite image on-and-off using the check box on the left hand side. Additional control points can be added to tie in areas that are not registered correctly.

Reasons for High RMS values Points with high RMS values can be deleted and re-selected if they are mis-placed. If the point or pixels are deemed correctly identified in both images and the high RMS value persist, it may be due to variable terrain/relief. The order of the transformation can be changed. In areas with large relief, a higher order polynomial transformation is justified (2nd, 3rd order).

Final Thoughts Vector layers could have been used as the base: Roads, building footprints Limitations: Transformations available are limited in ArcGIS. Image geo-referencing and rectification are easier done using image processing software.