Fundamentals of orthorectifying a raster dataset

When you receive an unrectified image, there is distortion across the image caused by distortions from the sensor and the earth's terrain. For example, since satellite imagery can be collected by scanning side to side along a path, this movement while collecting means that a spatially adjacent part of an image may have been collected from a nonadjacent part of the sensor, and although there are algorithms that bring this together before it is delivered to you, it can be improved. By orthorectifying an image, the distortions are geometrically removed, creating a planimetric image at every location with consistent scale across all parts of the image. In other words, orthorectification is the process of stretching the image to match the spatial accuracy of a map by considering location, elevation, and sensor information.

You can produce an accurately orthorectified raster dataset using the rational polynomial coefficients (RPCs), if they are provided by the vendor, and an accurate digital elevation model (DEM). You can improve the spatial location in the image of a pixel by applying the information within the associated RPC file, that is, the coefficient to the latitude, longitude, and height value of the pixel.

Companies such as DigitalGlobe or GeoEye deliver some of their images with the RPCs (essentially, the abstraction of their proprietary camera model), which can be used to geometrically correct the image. Subsequently, by including elevation information, these images can be orthorectified. You can use a DEM to perform an orthorectification, or you can specify an average elevation if no DEM is available. In areas of diverse terrain, it is recommended that you use a high-resolution DEM to achieve the most accurate orthorectification.

Images from DigitalGlobe store RPC information in a <file name>.rpb file, and GeoEye offers images with RPC information stored in a <file name>_rpc.txt file. ArcGIS automatically reads RPC information in the associated text file (if it can be found next to the image file) and can apply the RPC transformation (image correction) on the fly when displaying it in ArcMap. This oriented image can also be orthorectified on the fly in ArcMap by applying elevation information. You can use a DEM to perform orthorectification or use an average elevation if no DEM is available for the same area.

There are several ways to apply an orthorectification to a raster:

• You can orthorectify a raster layer in ArcMap by modifying the layer properties to use the RPCs and a DEM.
• You can orthorectify a raster layer in ArcMap using the Orthorectify button on the Image Analysis Window.
• You can permanently orthorectify your image using the Create Ortho-corrected Raster Dataset tool or by exporting your orthorectified on-the-fly raster dataset layer using the ArcMap Export Raster dialog box, found by right-clicking the raster in the table of contents.
• You can add the Geometric function to a raster within a mosaic dataset.

Using a DEM

You can use a DEM stored as a raster dataset (in any ArcGIS supported raster format), stored in a raster catalog, a mosaic dataset, an image service, or WCS service.

If the format has a NoData value this is supported. You can verify the NoData value in the raster properties of your raster dataset. If you need to convert a value to NoData, use the Copy Raster tool and define a value for the NoData Value parameter. This tool will output a new raster dataset. If you do not want to create a new raster dataset, you can add your DEM to a mosaic dataset and use the Define Mosaic Dataset NoData tool to define the NoData value. Using this method you can define more than one value to be interpreted as NoData.

If you have multiple DEM raster datasets you can add them to a mosaic dataset to create a single dataset that can be used as the DEM.