Recomputing footprints based on geometry

Recomputing footprints by geometry redefines a footprint for each raster dataset by considering the warping applied to the pixels as they are georeferenced. For rasters that are georeferenced with no rotation, this process is equivalent to determining the envelope and can be used to reset the extents. This feature is very helpful, especially when you have raster datasets with complex georeferencing, as it can accurately determine the edges and therefore eliminate slivers that may appear along their edges in undulating terrain. It should also be run on raster datasets if their georeferencing has been changed.

Recomputing footprints by geometry is calculated at the raster level by transforming a set of points along the edge of the raster dataset from the raster coordinate system into the ground coordinate system through the different transformations and projections that may be applied. The vertices of the footprint polygon are projected into the image service spatial reference system. This operation is based only on the geometry of the pixels. Processes that affect the radiometry of the service have no effect on the results.

The Recompute Footprint By Geometry dialog box has two tabs: General and Advanced. The following image type options are provided on the General tab. The option you choose in the General Tab updates parameters in the Advanced tab that are used by this process.

• Rectified sheet—Use when the raster dataset represents a rectangular area that may or may not be rotated, and the edges can be assumed to be straight.
• Projected sheet—Use when the raster dataset is being reprojected through a transform that would result in the edges of the sheet becoming curved.
• Orthorectified image in flat terrain—Use when the raster dataset is being orthorectified in a flat to moderately flat terrain—basically, in an area that encounters less change in elevation.
• Orthorectified image in hilly terrain—Use when the raster dataset is being orthorectified in an area that encounters large change in elevation and so requires more points along the edges to correctly model the terrain.

Using the Advanced tab, you can view or set the following parameters:

• Approximate number of vertices—Number of vertices that can be used to create the new footprint. This value should be increased for more complicated raster datasets, such as ones with curved edges. For a simple rectified sheet, this can be set to 4.
• Shrink columns—Percentage that the polygon size should be reduced in width. This value should be increased when there is a border within the raster dataset of invalid data, such as the frame in scanned aerial film. Instead of using reprojection points on the edge of the raster, the points will be moved in by the specified number of columns.
  提示:

  You can enter positive or negative shrink values; however, negative shrink values are not recommended, as they will create gaps between the rasters and the footprints. Use a "-" before a negative entry and no symbol before a positive value.

  提示:

  When working with pan-sharpened satellite imagery, it is recommended that you specify a shrink value of about 1 percent in the columns and 2 or 3 percent in the rows. The is because the multispectral image often does not correspond exactly to the panchromatic image on which the footprint is calculated, which would result in some fringing at the edge of the scenes.

• Shrink rows—Percentage that the polygon size should be reduced in height. This value should be increased when there is a border within the raster dataset of invalid data.
• Skip derived images—By default, this option is checked, which implies that the derived images will be skipped while recomputing the footprints. When you uncheck this option, the footprints of the derived images will be recomputed.

步骤：

1. Open an existing image service definition or use one that is already open in ArcMap.
2. Right-click the Footprint layer in the table of contents and click Recompute Footprint > By Geometry.

   The Recompute Footprint By Geometry dialog box is displayed with default values for each of the parameters.

3. Click the Image type drop-down arrow and click one of the options.
4. Optionally, click the Advanced tab and make changes to the parameters.
5. Click OK.