How To: Work with raster products in ArcGIS for Desktop, an introduction
The article provides a basic overview of raster products, their structure, and how to begin working with them in ArcGIS for Desktop.
Raster products are produced by vendors for specific sensor types, such as Pleiades, Spot, DMCii, and Landsat 8. They contain all the necessary components for the proper display and orientation of the imagery collected by these sensors. There are typically different product levels for each sensor that vary spectrally, for example, panchromatic, multiband, pansharpened, etc., and by the level of georeferencing, for example, raw non-georeferenced to orthorectified. There may be other processing differences between product levels. When working with raster products, be aware of the following details:
• The sensor type
• The spectral type
• The georeferencing level
If the product is a familiar one, some of this information can usually be determined from the file name, but keep in mind that different vendor products use different designations. For example, 'MS' in the file name of a Pleiades product indicates it is a multispectral product and '1A' indicates it is raw and not georeferenced.
If unfamiliar with a particular raster product, the product level is usually provided in the HTM- or HTML-based metadata file found in the product folder. Once the product level is determined, it may be necessary to visit the vendor’s website to obtain more details about the product being used. Some vendors provide very detailed documentation on their products which can be a valuable resource. The vendor websites are also great places for obtaining free samples of the raster products, including the different product levels that are available.
A key point to keep in mind is that Esri documentation is very limited regarding specific raster products, so it may be necessary to locate and use the vendor-developed documentation to fully understand and correctly use a raster product. Additionally, different versions of ArcGIS for Desktop support different raster products; consult the relevant version documentation when working with new raster products. For example, which raster products are supported in 10.2.x can be identified by reviewing the list of supported raster and image data formats.
Raster products are provided by the vendor as a set of files in a product folder. When browsing to the product folder in Windows Explorer, there are typically a variety of files included. For example, the figure below shows the contents of one of the Pleiades multispectral product folders.
As can be seen, there are several files and even some additional folders, but we are typically only concerned with the four highlighted files. This example is for a Pleiades product, but a similar set of files are associated with most raster products. Below are descriptions for each of the files highlighted by color.
This is the DIM metadata XML file for a Pleiades product. Depending on the vendor and product, it may have a different name but they all serve the same purpose. This metadata XML file contains the instructions needed to display the raster product in ArcGIS software. It is essentially a metadata file that ArcGIS or other applications read to determine which files to use in the product folder and how to use them. It contains a reference to the actual imagery but also other relevant files, like the RPC file. When this file is opened in an advanced text editor like WordPad or an XML editor, the contents of this file can be viewed, and it is apparent that it contains a considerable amount of information.
What is most important to know about the metadata XML file is that this is the file that should be worked with when using a raster product. In other words, to use a raster product in ArcMap, the metadata XML file must be added and not the image file, for example, TIFF, JP2, etc. The image file included in the raster product folder is not the raster product; it is just an image file like all other image files. If the image file is added to ArcMap, the raster product is not being used and it appears differently, and likely has inaccurate georeferencing, and does not have the advertised capabilities of the raster product. Even though the raster product metadata file is an XML file, it actually shows up as a raster dataset in Catalog. It is easy to identify because it appears with a small satellite icon.
When adding raster products to a mosaic dataset, if the specific raster product under the 'Raster Type' parameter is specified, it adds the metadata XML file and not the image file. Which file is being used for a specific raster product can be seen by reviewing the 'Filter' parameter in the Raster Type Properties dialog.
This is the image file that was collected by the sensor. As mentioned above, it is not the raster product and is actually only one of the components that make up the raster product. By itself, there is nothing special about this file and it behaves just like any other image file. This file contains no reference to the RPC file, which is needed for orthorectification. Actually, this file may not have any georeferencing information associated directly with it or only very coarse georeferencing, and when added directly to ArcMap it may appear in the wrong location. Additionally, this file may look different than the raster product when added directly to ArcMap because the metadata XML file contains additional information on rendering the raster product. Unless there is a specific need, there is no reason to work directly with the image file when using a raster product.
This HTM file is basically a simplified and easier to read metadata file that opens in a web browser. It contains basic metadata about the file but much less than what is found in the metadata XML file. It is useful for quickly obtaining relevant metadata about a specific product, including the product level.
This is the Rational Polynomial Coefficient (RPC) file, which contains all the necessary information to use in conjunction with a DEM to orthorectify imagery. In the above Pleiades example, it is an XML file, but it can also be a TXT file or a proprietary vendor file type, for example, .rpb. Like the DIM metadata XML file, its contents can also be viewed in a text or XML editor. More information can be obtained about how RPC files are used in the help topic: 'Fundamentals of orthorectifying a raster dataset'. When the raster product metadata file is added to ArcMap or a mosaic dataset, the RPC information is automatically pulled from the RPC file and used to orthorectify the image as long as the image and RPC file are stored in the same folder (as in the above example).
If it is necessary to use a raster product in a mosaic dataset, it is important to specify the raster product when creating the mosaic dataset and adding rasters to ensure proper display of the imagery:
1. In the Create Mosaic Dataset tool, specify the appropriate product under the 'Product Definition' parameter.
2. In the Add Rasters To Mosaic Dataset tool, specify the appropriate raster product under the 'Raster Type' parameter.