Recalculating footprints radiometrically

This topic applies to ArcEditor and ArcInfo only.

When you need to recalculate the footprint to remove those pixel values you don't want in your mosaic dataset, you can use the Build Footprints tool. For example, your footprint is originally the entire dimension of the image, but because it was rotated, you want it to reflect the actual valid pixel values of your images.

There are a number of options on the Build Footprints tool that can be modified that will affect the footprint output. For example, there may be a range of pixel values on the high and low ends of your data values that you don't want included. Normally, these pixel values can easily be defined, but if the data is stored with lossy compressions, the values will be a bit fuzzy. For example, instead of all values of 0, your values may be 0–3.

Recommended parameter settings

The table below defines the various parameters that can be altered to radiometrically adjust the footprints.

Parameters	Description
Minimum data value	The lowest pixel value representing valid image data. This value is determined by the bit depth of the raster dataset. For example, with 8 bit data, the values can range from 0 to 255. A value around 0 represents very dark colors, like black border pixels. When you specify 1, then the only value less than 1 is 0, so all 0 values will be considered invalid data and will be removed from the perimeter of the footprint. If the imagery is compressed using a lossy compression method, then you should define a value slightly greater than 1 to remove all the black pixels. When dark areas, such as shadows, have been incorrectly removed from the footprint, this value should be decreased.
Maximum data value	Highest value representing valid data. This value is determined by the bit depth of the raster dataset. For example, with 8-bit data, the values can range from 0 to 255. A value around 255 represents very bright colors, such as white clouds and snow. If you specify 245, then all values between 246 and 255 will be removed from the perimeter of the footprint.
Approximate number of vertices	Approximate number of vertices with which the new footprint polygon will be created. The minimum value is 4 and the maximum value is 10,000. The greater this value, the more accurate and irregular the polygon and the longer the processing time. A value of -1 will show all the vertices in the footprint, therefore, your polygon footprint will not be generalized.
Shrink distance	Distance value specified in the units of the mosaic dataset's coordinate system by which the overall polygon will be reduced in size. Shrinking of the polygon is used to counteract effects of lossy compression, which causes edges of the image to overlap into NoData areas.
Request size	The size to which the raster will be resampled when it is examined using this tool. The value (such as 2,000) defines the dimension using rows and columns. You can increase or decrease this value based on the complexity of your raster data. Greater image resolution provides more detail in the raster dataset and thereby increases the processing time. A value of -1 will not resample the footprint, therefore it will compute the footprint at the native pixel size.
Minimum region size	Determines a filter used to remove holes created in the footprint. This value is specified in pixels, and it is directly related to the Request Size, not to the pixel resolution of the source raster.
Maintain sheet edges	Check when using raster datasets that have been tiled and are butt joined (or line up along the seams with little to no overlap). An analysis of the image edges is performed so that the sheet edges are not removed.

The tables below define various types of datasets that sometimes have NoData borders and the recommended parameter settings.

Caution:

The shrink distance below is defined by pixels, but the units of this parameter must be entered in the units of the mosaic dataset's coordinate system. Therefore, if the units are meters, each pixel is 10 meters, and the recommended shrink distance is 3, you should enter 30.

Regular and clipped to sheets

Used for datasets where the data pixels in each image form a rotated rectangular area. The resulting imagery will be clipped to a new sheet or tile. Such datasets are typically created by reprojection of images or scenes, then cut to map sheets or tiles with little or no overlap. The parameters are set such that the footprint will contain only a few vertices. An analysis of the edges is performed to maintain the sheet or tile boundaries.

Parameter	No or lossless compression	Lossy compressed
Minimum data value	1	10
Maximum data value	254	245
Approximate number of vertices	4	4
Shrink distance	2 pixels	6 pixels
Request size	1000	1000
Maintain sheet edges	Yes	Yes

Irregular and clipped to sheets

Used for datasets where the data pixels do not form rectangular areas. The resulting imagery can be cut into map sheets or tiles. Such datasets are typical for imagery along pipelines or other linear features. Here, the pixel areas cover the linear feature, and the images are then mosaicked and cut into tiles. The parameters are set to allow a larger number of vertices to define the border. An analysis of edges of the resulting footprint is performed to maintain the sheet or tile boundaries.

Parameter	No or lossless compression	Lossy compressed
Minimum data value	1	10
Maximum data value	254	245
Approximate number of vertices	30	30
Shrink distance	6 pixels	6 pixels
Request size	1500	1500
Maintain sheet edges	Yes	Yes

Rotated rectangle

Used for images that form a rotated rectangle. Such datasets are typically created when individual scenes or map sheets have been rotated and the sides of the footprint remain straight. The parameters are set to define the footprint only by four vertices.

Parameter	No or lossless compression	Lossy compressed
Minimum data value	1	1
Maximum data value	254	245
Approximate number of vertices	4	4
Shrink distance	2 pixels	6 pixels
Request size	1000	1000
Maintain sheet edges	No	No

Reprojected rectangle

Used for rotated images that have been reprojected, which form rectangular footprints with curved edges. Such datasets are typically created when individual scenes or map sheets have been reprojected. The parameters are set to define the footprint with a sufficient number of vertices to represent the curves.

Parameter	No or lossless compression	Lossy compressed
Minimum data value	1	10
Maximum data value	254	245
Approximate number of vertices	20	20
Shrink distance	2 pixels	6 pixels
Request size	2000	2000
Maintain sheet edges	No	No

Orthorectified image in flat terrain

Used when the origin of the raster dataset is a scene or image that has been orthorectified to a flat terrain. The edges of such images form simple curves caused by smooth changes in the elevation.

Parameter	No or lossless compression	Lossy compressed
Minimum data value	1	10
Maximum data value	254	245
Approximate number of vertices	30	30
Shrink distance	9 pixels	9 pixels
Request size	2000	2000
Maintain sheet edges	No	No

Orthorectified image in hilly terrain

Used when the raster dataset is being orthorectified in an area that encounters large changes in elevation. The edges of such images are irregular, caused by sudden changes in the elevation. More vertices are required to define such footprints.

Parameter	No or lossless compression	Lossy compressed
Minimum data value	1	10
Maximum data value	254	245
Approximate number of vertices	200	200
Shrink distance	9 pixels	50 pixels
Request size	2000	2000
Maintain sheet edges	No	No

Minimum region size

A suggested default for this parameter was not specified in the tables above because of the potential complexity of this value. You need to consider what the raster data is and how detailed you require the footprint to be.

Your footprint is designed to define the area of the raster dataset you want to view. The footprint is typically the extent of the raster dataset; however, it can be modified so a user cannot view part of the raster dataset.

The minimum and maximum data values are used to specify the valid data. Outside those values is a range of pixel values that will be used to create a region, or continuous feature, in the raster dataset that will create a "hole" in the footprint polygon. For example, if you have a raster dataset with pixel values from 0 to 255, you can define the valid range as 10–255. Therefore, the pixels from 0 to 9 will result in holes in the footprint. However, your image may have pixels from 0 to 9 that you want to retain because they represent valid features. You need to consider what these may be. For example, if the rooftop of a home is valid but is likely to have pixel values from 0 to 9, you need to ensure these do not result in holes in the footprint; whereas larger features, such as large clouds, do.

The area of the hole is compared to the area computed using the minimum region size. If the area of the hole is smaller than the area computed by minimum region size, the hole is removed. Minimum region size ensures that only reasonably large features are eliminated from the footprint by having them remain as holes in the computed geometry. All candidate holes that are smaller in area than that area denoted by this parameter are removed; that is, they no longer appear as holes in the computed footprint. thus ensuring such tiny features are not clipped from the dataset.

In the diagram below, the gray pixel values represent the valid data values. The orange pixels represent three regions in this raster containing values you potentially want excluded. The yellow boxes represent the area defined by the minimum region size. The two orange regions on the left side are smaller than the minimum region size, therefore, the holes in the footprint polygon will be removed and the pixels are not excluded. However, the large orange region on the right side is larger than the minimum region size, therefore, the hole in the footprint polygon will not be removed and the pixels will be excluded from the mosaicked image in the mosaic dataset.

The request size also needs to be considered when specifying the minimum region size, because the request size determines the resolution or detail in the raster used to recompute the footprint. Normally, you will use a request size that is smaller than the dimensions of your original source raster. This will impact your region size. For example, if you want to preserve the rooftop area, you need to consider the roof's pixel dimensions in the source raster and its dimensions in the request size. Therefore, if the roofs are 50 x 50 pixels in the source raster and the request size represents a raster that has 50 percent fewer pixels in its x and y dimensions, the rooftops may be represented by a feature that is only 25 x 25 pixels in dimension. You need to be cognizant of the request size versus the minimum region size so you don't exclude features you want to retain but still exclude any desired features by creating holes in the footprints. In other words, you need to define the correct size to remove the small holes but retain the larger holes.