Mosaic dataset footprints
The footprints contain the outline of each raster within the mosaic dataset. This outline is not always the extent of each raster dataset but can be the extent of the valid raster data within the dataset. For example, in some satellite imagery, such as Landsat, the extent of raster data is not rectangular as suggested by the file extents. It is, in fact, contained within a skewed rectangle in the dataset, and the other pixel values surrounding this skewed rectangle are null data. The example below displays a Landsat 1G scene and the footprint (green polygon) that would be derived:
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The footprint is created using the spatial reference of the mosaic dataset, which may be different from the source raster datasets. It is generated based on the raster type chosen while adding raster data to the mosaic dataset. For example, when you choose to add a TIFF file using the raster dataset raster type, it creates a footprint for the entire raster dataset. On the other hand, when you choose to add a dataset using the Landsat raster type (even though it may be stored as a TIFF), it generates a footprint that excludes the null data.
Each raster dataset can be clipped by the footprint as it is mosaicked, or you can choose to ignore the footprints, so all the pixels will be displayed. You can change this setting on the Mosaic Dataset Properties dialog box. Click the Defaults tab and change the Always Clip the Raster to its Footprint property to Yes or No.
Areas of NoData are a typical example of content excluded by the footprint's shape, but the footprint can also be used to clip off the collars of scanned maps or remove the black frame and fiducial marks around georeferenced, scanned aerial photography. The footprint for each raster dataset is saved as a polygon within the mosaic dataset. You can modify the footprints using the Build Footprints tool to perform some systematic calculations, or you can use the standard ArcMap feature editing tools.
NoData values can also be removed from display in a mosaic dataset using the Define Mosaic Dataset NoData tool. This tool will allow you to specify more than one NoData value.
When you display a mosaic dataset, it will display the mosaicked image, but if it cannot, it will display the footprints. By default, the footprints will be displayed as a hollow green polygon, but the symbology can be changed using the standard ArcMap layer symbology properties.
Recalculating footprints
This section applies to ArcGIS for Desktop Standard and ArcGIS for Desktop Advanced only.
When adding raster datasets, the footprints may not be defined correctly because sufficient metadata about the extents may not be available or they may contain pixels that represent NoData. Or perhaps you want to modify them so specific features in the data will not be displayed.
You can use the editing tools within ArcMap to modify the shape of each footprint polygon, or you can use the Build Footprints tool.
Caution:When recalculating the footprints, you may be increasing the number of vertices for each footprint. Too many vertices can affect the display performance. You can use the simplification method to reduce the number of vertices.
Recalculating footprints by radiometry
When you recalculate the footprint of a raster dataset according to its radiometry, this redefines the shape of the footprint based on a pixel value range. This option is generally used to exclude border areas, which do not define valid data. Such border areas—mostly in black or white—occur on rotated raster datasets or scenes that were rectified but not mosaicked. The footprint is recalculated based on the radiometry of the border pixel value.
This calculation is performed with the Build Footprints tool using the Radiometry method.
To learn about the recommended settings, see Recalculating footprints radiometrically.
Clipping
You can clip the footprints to the boundary (if its shape has been modified) or another polygon feature class using the following steps:
- Click the Editor drop-down menu on the Editing toolbar and click Start Editing.
- Using the Edit tool
on the Editor toolbar, click to select the boundary or feature class to which you will be clipping the footprints. - Select the Footprints layer in the table of contents.
- Click the Editor drop-down menu on the Editor toolbar and click Clip.
- Click the Preserve the area that intersects radio button and click OK.
Shrinking
You can use the Build Footprints tool to shrink the footprints by specifying a value for the Shrink Distance parameter. The value specified must be in the units of the mosaic dataset's coordinate system.
You can shrink the footprint in two ways: as part of the operation of recalculating the footprint by radiometry (where you specify a minimum and maximum data value), or by shrinking but not altering the footprint shape by radiometry. To do the latter, you need to check the option to Reset footprints to default. This will calculate the shape of the footprint according to the transformation being applied (such as a simple affine or more complex orthorectification), then shrink the shape by the specified Shrink Distance.
Updating the transformation
If you modify the transformation being applied to the raster dataset or update the DEM used in an orthorectification, you should update the footprint to reflect the change in the transformation. To do this, use the Build Footprints tool and set the Computation Method parameter to Geometry.
Undoing the footprint recalculation
If the edits you've made to the footprints turn out to be incorrect, you do not need to remove and then re-add your data. You can use the Build Footprints tool and set the Computation Method parameter to Geometry.
Replacing footprints
The Import Mosaic Dataset Geometry tool uses a join operation to replace the footprints of the mosaic dataset with the footprints polygons in a feature class. To perform a join, you need an attribute field that is common to both files.
This tool can also be used to replace the boundary or seamline polygons.
Exporting footprints
If you need to export the mosaic dataset's footprints, use the Copy Features tool. This will create a polygon feature class containing all the footprints and the associated fields from the mosaic dataset's attribute table.