DEM Extraction module


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Description


The DEM Extraction module generates a digital surface model (DSM) from overlapping stereo satellite image pairs. The module also provides an option for converting the DSM to a digital terrain model (DTM). If you have tri-stereo satellite imagery, then consider using the DEM Extraction Tri-stereo module instead.
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Parameters


Name Caption
Input Scenes Input scenes folder
Output Folder Output folder
Output DEM Base Name Output DEM base name
Save Intermediate Files Save intermediate files
Overwrite Results Overwrite existing results
Send Email Email notification settings
Output Pixel Size Output pixel size
Custom Size Custom size of output pixel
Output Map Units Output projection
Elevation Reference/Datum Elevation model
Failed Pixel Value Failed pixel value
Background Elevation DEM background elevation value
Pair Selection Method Epipolar pair selection method
Minimum Overlap Minimum overlap percentage
Tri-stereo Look Direction Pair Look-direction pair to use from tri-stereo data
Use Alternative Pair Alternative look-direction pair
Input Channels Input channels
Extraction Method DSM-extraction method
Epipolar Downsampling Factor Downsampling factor
Clip Amount DEM clipping
Clip Units Clipping units
SGM Extraction Interval Extraction interval for SGM
Cost Function Cost function for SGM extraction method
Penalties Penalty values for cost function
Segment Size Threshold at which to eliminate segments
Semi-global Filter Level of SGM filtering
Pattern Suppression Suppress pattern blunders
Track Epipolar Lines Whether to track epipolar lines
NCC Extraction Interval Extraction interval for DSM
Terrain Type Type of terrain
DEM Detail DEM level of detail
Estimated Min & Max Elevation Estimated minimum and maximum elevation
Apply Wallis Filter Apply Wallis filter before DEM extraction
Create DTM from DSM Create DTM from DSM
Output DTM Pixel Size Size of pixel in output DTM
Filter Type Filter to apply during conversion
Create DEM Mosaic Create DEM mosaic
DEM-Mosaic Method Method to create the DEM mosaic
Tile Specification Mosaic tile specification
Height Height of the mosaic tile, in pixels
Width Width of the mosaic tile, in pixels
Vertical Overlap Vertical overlap between tiles
Horizontal Overlap Horizontal overlap between tiles
Delete Empty Tiles Whether to delete empty tiles
Clean Up Building Edges Straighten building edges
Edge-Cleaning Filter Size Size of edge-cleaning filter
Cutline Method Cutline method
Cutline Method Extra Options Cutline options
Auto Constrain Constrain cutlines to image centers
Factor Auto Constrain Factor value
Blend Width Cutline blend width
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Parameter descriptions

Input Scenes

The name of the folder that contains a valid CATALYST Professional OrthoEngine project file (oe.prj), valid GDB-supported images, or a text file of images to be processed.

When only a folder name is specified, the module processes all recognized .tif and .pix image files in that folder.

This parameter can be specified by using any of the following:

Output Folder

The path and name of the folder to which to write the output files.

Output DEM Base Name

The base name of the output digital surface model (DSM) and digital terrain model (DTM) files. This name is prepended to all of the output DSM and DTM files.

If no value is specified for this parameter, the default digital elevation model (DEM) base name is dem.

Save Intermediate Files

Select this check box to save any supporting files created during processing.

By keeping the files, you can use them to analyze intermediate results and, if necessary, fix any minor issues. You can then restart the job without having to regenerate all the supporting data, thereby reducing processing time.

Note: Intermediate files can consume a large quantity of disk space, thus it is a good idea to delete them when they are no longer needed.

Overwrite Results

Select this check box to overwrite the existing output files, if any exist. If this check box is left clear, and an output file exists in the relevant folder, the status of the job displays a message informing you of the existence and name of the output file. The message is also written to the event log of the job.

Send Email

If necessary, you can set up CATALYST Enterprise to send an email notification on job start and job completion.

With this check box selected, an email message is sent to each address specified in the Email Addresses box after the job starts and on completion.

You can specify one or more addresses, and each must be separated by a comma or a semi-colon. The email address of the user currently logged in displays by default.

Output Pixel Size

The size of the output pixel in the DSM being created and, if applicable, the DTM. The size of the output pixel can be created either as a multiple of the ground sample distance (GSD) or you can specify a custom size.

Select one of the following:

Custom Size

Available when you select Custom for Output Pixel Size, the custom pixel size of the output DSM, DTM, or both.

For example, when Output Map Units is UTM, the output pixel size must be in meters. With Long/Lat, the output pixel size must be in decimal degrees.

Output Map Units

The projection of the output imagery.

The value of this parameter must be in the PCI Projection String format.

The standard definitions are:

If you do not specify a value for Output Map Units, the map unit of the input image is used for the output image. If the input data is a variety of map units, the map unit of each output image is that of its corresponding input image. In such a case, it is recommended that you specify the output map units.

You can also specify the label of a projection defined in the userproj.txt file.

Elevation Reference/Datum

The vertical reference for the elevation values in the source digital elevation model (DEM), or for the constant value of the Elevation Offset and Elevation Scale parameters, when available, and specified.

Available values are as follows:

Failed Pixel Value

The value attributed to pixels containing distortion or incorrect elevation values.

If no value is specified, a default value of -500 is applied.

Background Elevation

The pixel value representing background elevation (pixels where no elevation value is present) in the input digital elevation model (DEM).

If no value is specified for this parameter, the module checks for ELEVATION_BACKGROUND or NO_DATA_VALUE metadata tags, first at the channel level, and then at the file level. If a value is neither specified nor found in the metadata, the default value of -32768 is used for 16S and 32R data, 65535 for 16U, and 0 for 8U.

Pair Selection Method

The method to use to form epipolar pairs.

Available methods are:
Note: Not all methods are available in all modules.

Minimum Overlap

The minimum percentage amount of overlap between two images for them to be considered a valid pair. If no value is specified, a system default of 50 (percent) is applied.

The first step in the pairing is to evaluate every pair to determine if the overlap is greater or equal to the value of this parameter. For each image pair, A and B, two overlap values are calculated: A/B and B/A. Pairs are rejected when the minimum of A/B and B/A is less than the value MINPC / 100.

Examples:

Image Overlaps

CAUTION:
This parameter is ignored with some values of the Pair Selection Method parameter.

Tri-stereo Look Direction Pair

The look-direction pair to use from tri-stereo data. The other look directions are eliminated (filtered out).

Available options are:

Note: If you have tri-stereo data consider using the DEM Extraction Tri-stereo module.

Use Alternative Pair

Select this check box to use any available look-direction pair should the pair you have specified for Tri-stereo Look Direction Pair be unavailable.

Input Channels

The input image channels to use to generate epipolar image pairs.

Extraction Method

The method to use to extract the DSM. Each method is an algorithm, either Normalized Cross-Correlation (NCC) or Semi-global Matching (SGM), that is used to determine the elevation.

You can select from the following:

Epipolar Downsampling Factor

The downsampling interval, in pixels, for the created epipolar image. The value you specify controls the number of pixels from the raw image used to generate the epipolar image.

Clip Amount

The amount of data to clip from the epipolar digital elevation models (DEM) before merging and geocoding.

This parameter specifies the amount of the actual image data that is clipped, either as a number of pixels or as a percentage of the image size.

You can select between Pixels or Percent using the Clip Units parameter.

You can enter up to two values:

For example:

If Clip Units is set to Pixels:
If Clip Units is set to Percent:

The default is blank, which means no clipping occurs. The maximum percentage value is 49 percent.

Clip Units

The unit to use for the value of the Clip Amount parameter.

Available options are:

When you specify a value for the Clip Amount parameter, this parameter is mandatory; otherwise, it is not required.

SGM Extraction Interval

The extraction interval for the DSM.

You can specify a value of 1, 2, 4, 8, 16, or 32. Typically, a value of 1 (full-resolution) or 2 (half-resolution) is used for high detail. You can use a higher value, such as 8, to generate a quick preview, which you can use to check quality before generating the higher-resolution DSM.

Cost Function

Available when Semi-global Matching is selected for Extraction Method, the cost function to use for SGM.

You can select from the following:

Penalties

Available when Semi-global Matching is selected for Extraction Method, the penalty value based on that selected for Cost Function.

You can enter the value of the penalty as follows:

Segment Size

Available when Semi-global Matching is selected for Extraction Method, the threshold at which to eliminate segments and interpolate values of the digital elevation model (DEM) from the surrounding data. A segment is a small, isolated group of matching pixels.

Typically, values ranging from 16 through 128 provide satisfactory results in most cases. The default value is 64. A value of zero (0) switches off the operation.

Semi-global Filter

The level of SGM filtering to apply to the output DSM.

You can select from the following: Filtering is applied in successive applications of noise, median, and smoothing filters.

Pattern Suppression

The level of pattern suppression to apply.

You can select from the following:

Pattern suppression helps to reduce or eliminate blunders in the output digital surface model (DSM) with imagery that contains repeating patterns, such as rows of crops in an agricultural field or tiles on building roofs.

The higher the level of suppression, the greater the likelihood that degradation of other features occurs, such as loss of high building tops or small trees. If such features are important, consider selecting Low or doing no pattern suppression whatsoever. You may also want to consider manually removing the blunders or using pattern suppression only on problem images.

Track Epipolar Lines

Available only when you select a semi-global filter, select this check box to automatically track errors in epipolar lines. Tracking may increase quality, but may also increase computing time.

NCC Extraction Interval

The extraction interval for the DSM.

You can specify a value ranging from 1 through 32. Typically, a value of 1 (full-resolution) or 2 (half-resolution) is used for high detail. You can use a higher value, such as 8, to generate a quick preview, which you can use to check quality before generating the higher-resolution DSM.

Terrain Type

The types of terrain in the scenes.

Available options are:

DEM Detail

The level of detail to include in the output digital elevation model (DEM). A higher level of detail requires more computation time.

Acceptable values are:

Estimated Min & Max Elevation

The search radius at the initial (coarsest) resolution is derived from the elevation range within the scene, specified by the Estimated Min & Max Elevation parameter, or extracted automatically. If the range is too narrow, some of the matches may not be found, resulting in failed values. If the range is too wide, some invalid matches may be accepted, lowering the quality of the DEM.

This parameter is required only if METER or FEET is specified as the value for Output Map Units.

Apply Wallis Filter

The Wallis filter can to be useful for desert data or areas with significant shadows, such as mountainous areas or urban scenes.

When this check box is selected, a Wallis filter is applied to the input epipolar images before extracting the DEM. When the check box is clear, no filter is applied; this is the default option.

Create DTM from DSM

Allows you to select whether to convert the output digital surface model (DSM) to a bare-Earth digital terrain model (DTM).

A DSM can have elevation values that reflect the top of canopy or top of buildings. However, for many processes, such as orthorectification, it is necessary to have a DTM that reflects the height of the bare Earth.

If you need more control over how the output DSM is converted to a DTM, then leave this parameter unchecked and use the DEM Convert (DSM to DTM) module after this job has completed.

Output DTM Pixel Size

The size of a pixel in the output digital terrain model (DTM).

The size is a multiple of the input DSM pixel size. The DSM is resampled accordingly.

You can select from the following:

If you need more control over how the output DSM is converted to a DTM, then uncheck Create DTM from DSM and use the DEM Convert (DSM to DTM) module after this job has completed.

Filter Type

The type of filter to apply when converting to the digital terrain model (DTM).

For best results, select from the following the filter that best describes your data set:

If you need more control over how the output DSM is converted to a DTM, then uncheck Create DTM from DSM and use the DEM Convert (DSM to DTM) module after this job has completed.

Tip: You can touch up areas of difficulty in the DTM, such as removing any remaining artifacts, in the DEM Editing window of CATALYST Professional Focus.

Create DEM Mosaic

With this check box selected, each extracted DSM pair is mosaicked into one or more DSMs. With check box cleared, a DSM file is created for each extracted DSM pair.

DEM-Mosaic Method

Select the method to use to create the DEM mosaic.

The available options are as follows:

Tile Specification

The mosaic tiling scheme to use for the output DEM.

Available schemes are:

Height

The height of the mosaic tile, in pixels.

The union of the extents of all of the source images is divided into a series of evenly sized and abutting rectangular tiles with the specified dimension.

Only tiles that actually intersect at least one of the source images is present in the output. The tiles at the far right and on the far bottom may overhang the extents of the source images. This is done to ensure that all tiles have the same dimensions.

TileID values are generated using the convention <column_number>_<row_number>. For example, the upper-left tile always has a TileID of "1_1", while the one immediately below it is "1_2", and so on.

For example:

10000

Width

The width of the mosaic tile, in pixels.

The union of the extents of all of the source images is divided into a series of evenly sized and abutting rectangular tiles with the specified dimension.

Only tiles that actually intersect at least one of the source images is present in the output. The tiles at the far right and on the far bottom may overhang the extents of the source images. This is done to ensure that all tiles have the same dimensions.

TileID values are generated using the convention <column_number>_<row_number>. For example, the upper-left tile always has a TileID of "1_1", while the one immediately below it is "1_2", and so on.

For example:

10000

Vertical Overlap

The vertical overlap of each tile, in pixels.

Note: The Width parameter value is considered sacrosanct and is always honored. Thus, setting the Vertical Overlap to something other than zero causes the mosaic tile positions to be adjusted, rather than changing the tile widths.

Horizontal Overlap

The horizontal overlap of each tile, in pixels.

Note: The Height parameter value is considered sacrosanct and is always honored. Thus, setting the Horizontal Overlap to something other than zero causes the mosaic tile positions to be adjusted, rather than changing the tile heights.

Delete Empty Tiles

Select whether to delete empty tiles. A tile is considered to be empty if all pixels in it have the value defined as NoData.

Clean Up Building Edges

With this check box selected, the filter straightens building edges in the DSM. When selected, you can use the default of 13 for Edge-Cleaning Filter Size, or select a new size.

While building edges are the intended target, the filter also affects trees and cliff edges. If the DSMs do not contain buildings, it is recommended that you clear the check box.

Edge-Cleaning Filter Size

The window size of the Clean Up Building Edges filter.

You can use the default of 13, or you can select a new size. Available sizes are odd integers ranging from 5 through 31. Larger sizes are well suited to buildings with long, straight edges, but may have an adverse affect on buildings with curved or complex edges. If the DSMs do not contain buildings, and you still want to use the filter, make sure the Clean Up Building Edges check box is selected, and then select a small-to-very-small window size.

Cutline Method

The cutline method used to generate polygons that enclose all the data from an image to be included in the output mosaic.

Supported methods are:

Cutline Method Extra Options

Additional options for Cutline Method.

You can specify options related to constraining polygons, which define regions where cutlines are allowed for each image, so that the generated cutlines do not cross the specified boundaries.

Values you specify for this parameter take precedence over Auto Constrain and Factor.

With all options for Cutline Method, except File Extents, you can constrain the cutlines by specifying the following extra options: 
[<vector_file>], [<field_name>], [<segment_number>]
Where:
The values in the attribute field, field_name, must correspond to the identity of one input source image. An input source image's identity is determined by:
  1. the value of its file level metadata tag: SourceID, or if that tag does not exist, then
  2. the base name, without the extension, of the input source image's file name.

If the field_name is not specified, MOSPREP searches the vector-segment attributes for a field named ImageSource. If the specified field name or ImageSource does not exist, an error occurs.

If the segment number is not specified, the algorithm uses the last segment from the vector file you specified. If the constraining polygon is larger than the image being processed, cutline generation is not constrained.

Auto Constrain

Select whether and how to automatically generate and apply constraint polygons when creating cutlines. Constraint polygons define regions where cutlines are allowed for each image, so that the generated cutlines do not cross the specified boundaries. You can opt to have them generated automatically based on the layout and arrangement of the images being mosaicked.

You can select from:

You can adjust the effect of Auto Constrain by specifying an appropriate percentage value for Factor.

When a constraining layer is specified for Cutline Method Extra Options, do not use Auto Constrain; that is, select No.

Factor

The Factor is a percentage by which to adjust effect of the Auto Constrain option.

This is a value between 1 and 100, with larger values causing more overlap between the generated constraint polygons; that is, the cutlines is less constrained.

Blend Width

The perpendicular distance from the cutlines over which image blending occurs.

Image blending is to average the gray value of each pixel in the blending strip along a cutline from both overlapping input images. If no value is specified for this parameter, no blending is performed.

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Details

General job details

Preprocessing requirements

Before running this module, the following requirements must be met to ensure the job processes successfully and produces accurate results:

Module details

The DEM Extraction module searches for valid images to use to generate stereo pairs according to the value of the Pair Selection Method parameter and if the convergence angle is between 1 and 140 degrees.

Qualified stereo pairs are then processed as a series of child jobs, one for each pair of stereo images. Each job first reprojects the stereo pair so that both images have a common orientation. The reprojected stereo images become a pair of epipolar images. The job then extracts a DSM from the epipolar pair and generates an epipolar DSM. Using epipolar images increases the speed of the correlation process and reduces the possibility of uncorrected matches.

All generated epipolar DSMs are then processed, reprojecting all epipolar DSMs and merging them into a single geocoded DEM.

If the Create DTM from DSM check box is selected, the module creates a DTM from the DSM for each pair. All DTMs are then merged into a single geocoded DTM.

Job results

On completion of processing, one or more files are created, matching the naming convention below:
  • dsm/[basename]_dsm_TILE_ID.pix: merged geocoded DSM file. These files are always created.
  • dtm/[basename]_dtm_TILE_ID.pix: merged geocoded DTM file. These files are created only when the Create DTM from DSM check box is selected.

Where the TILE_ID values are generated using the convention <column_number>_<row_number>. The upper-left tile always has a TILE_ID of 1_1, the one immediately below has the TILE_ID of 1_2, and so on.

Given the base name ottawa, the following tiles could be created:

ottawa_dsm_1_1.pix, ottawa_dsm_2_1.pix, ottawa_dsm_3_1.pix

ottawa_dsm_1_2.pix, ottawa_dsm_2_2.pix, ottawa_dsm_3_2.pix

ottawa_dsm_1_3.pix, ottawa_dsm_2_3.pix, ottawa_dsm_3_3.pix

Note: When Save Intermediate Files is checked, an intermediates folder is created in the specified output folder.
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