DEM Convert (DSM to DTM) module

Description

The DEM Convert (DSM to DTM) module transforms a raster digital surface model (DSM) or elevation into a bare-Earth digital terrain model (DTM). Filters are used to remove surface items, such as buildings and trees. Typically, you run the module on a DSM that was created automatically from stereo-image pairs.

Parameters

Name	Caption
Send Email	Email notification settings
Input Folder	Path and folder name of DSM data to convert
Output Folder	Output folder
Overwrite Results	Overwrite existing results
Output DTM Pixel Size	Size of pixel in output DTM
Custom Size	Custom size of pixel in output DTM
Filter Type	Filter to apply during conversion
Object Size	Size of objects to remove
Gradient Threshold	Gradient threshold (degrees)
Terrain Type	Type of terrain
Bump Filter	Filter for bump removal
Pit Filter	Filter for pit removal
Median Filter	Type and size of median filter
Clamp Filter	Size of clamp filter

Parameter descriptions

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.

Input Folder

The path and folder name of the DSMs to convert.

Output Folder

The path and folder name to which to write the converted DSMs. The folder is created automatically during processing.

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.

Output DTM Pixel Size

The size of the pixel in the output DTM. The size can be either a multiple of the input DSM-pixel size or a custom size you specify. The DSM will be resampled accordingly.

You can select from the following:

1 x DSM: Output pixel is that of the DSM pixel multiplied by 1.
1.5 x DSM: Output pixel is that of the DSM pixel multiplied by 1.5.
2 x DSM: Output pixel is that of the DSM pixel multiplied by 2.
2.5 x DSM: Output pixel is that of the DSM pixel multiplied by 2.5.
3 x DSM: Output pixel is that of the DSM pixel multiplied by 3.
3.5 x DSM: Output pixel is that of the DSM pixel multiplied by 3.5.
4 x DSM: Output pixel is that of the DSM pixel multiplied by 4.
Custom: Output pixel is a size you specify for Custom Size.
For information about the Custom option, see the description of Custom Size.

Custom Size

Available when you select Custom for Output DTM Pixel Size, the size of the pixel in the output DTM based on the geocoded map units of the DSM.

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:

Flat: Aggressively flattens protrusions in the terrain, such as buildings, hedges, and forests resulting in a very smooth DTM. This filter is best suited to flat areas with little-to-no natural-terrain protrusions.
Hilly: Flattens protrusions in the terrain while actively trying to preserve natural features. This filter can still remove buildings, but some artifacts may remain if they are detected as natural features. Hilly is best suited to scenes with elevations that vary naturally.
Hilly (Aggressive): Like Hilly, but with decreased gradients and a larger object size to more aggressively filter the DSM.
Flat (Low Detail/Satellite): Like Flat, but uses a different methodology to remove objects, because in a low-detail DSM, objects are not as sharp as in a high-detail DSM.
Hilly (Low Detail/Satellite): Like Hilly, but uses a different methodology to remove objects, because in a low-detail DSM, objects are not as sharp as in a high-detail DSM.
Custom: Provides you with complete control over the filtering to apply. That is, you can enter or select values for Object Size, Gradient Threshold, Terrain Type, Bump Filter, Pit Filter, Median Filter, and Clamp Filter, as necessary.
Note: Custom is available only with the DEM Convert (DSM to DTM) module.

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.

Object Size

The size of the filter to use to remove surface features. Typically, use a size relevant to the largest feature, such as the largest building to remove.

For example, to remove most buildings and small stands of trees, an object size ranging from 150 to 200 meters typically provides suitable results.

Therefore, to specify an object size of perhaps 175 meters, enter 175.

You may need to experiment to obtain the result you want. For example, a very-high value can remove desired natural features, such as small hills. The value you use can vary from DSM to DSM: the value that provides suitable results in one may not in another.

Note: The value you specify must be greater than zero.

Gradient Threshold

The degree of gradient that determines the flatness threshold.

Pixels with slopes below the defined threshold are treated as natural slopes and are not removed. This is useful for preserving hills and natural slopes. The higher the gradient threshold, the less likely you are to remove buildings with edges that are not well-defined; that is, those that do not have not sharp, crisp edges. The lower the value, the more likely you are the remove natural features of the terrain.

Lower values represent areas closest to horizontal (0 degrees); higher values represent areas closest to vertical (90 degrees).

Terrain Type

The types of terrain in the scenes.

Available options are:

Hilly: hilly terrain; this is the default value
Flat: gentle or flat terrain
Mountainous: mountainous terrain

Bump Filter

Available only with when Filter Type is Custom, use Bump Filter to specify how many bump filters to use to clean up small bumps left over from removing buildings and other large features.

You specify each bump filter you want to use as a pair of numbers, the size in pixels, and the gradient in degrees(slope). For example, to specify a bump filter of seven pixels with a gradient of 5 degrees, enter 7,5.

You can also use two pairs of values, such as 7,5,7,5.

Typically, you run bump filters and pit filters after the terrain-type filter; however, you can run them before the terrain filter by adding a minus sign (-) to the value; for example, -7,-5. Doing so can be effective in smoothing hills so they are not misinterpreted as buildings.

Pit Filter

Available only with when Filter Type is Custom, use Pit Filter specify how many bump filters to use to clean up pits and holes left over from removing buildings and other large features.

You specify each pit filter you want to use as a pair of numbers, the size in pixels, and the gradient in degrees(slope). For example, to specify a bump filter of seven pixels with a gradient of 5 degrees, enter 7,5.

You can also use two pairs of values, such as 7,5,7,5.

Typically, you run pit filters and bump filters after the terrain-type filter; however, you can run them before the terrain filter by adding a minus sign (-) to the value; for example, -7,-5. Doing so can be effective in smoothing hills so they are not misinterpreted as buildings.

Median Filter

Available only with when Filter Type is Custom, use Median Filter to specify the type and size of filter to apply as the last processing step. That is, Median Filter has two modes: hybrid and normal. Hybrid is used when the value is a positive number, or you can use normal mode by using a minus sign (-) before the value. With hybrid mode, smoothing of the final DTM is more pronounced in preserving sharp slopes while normal mode is less so.

You can use a value of 3 or greater; however, a larger filter may significantly increase processing time, particularly on a smaller area of interest. Typically, a value ranging from 7 through 11 provides suitable results.

Clamp Filter

Available only with when Filter Type is Custom, use Clamp Filter to specify a "clamp-type" filter. That is, with a clamp filter, a pixel cannot be altered in elevation by more than a percentage of the resolution of a pixel. For example, if the resolution of the DSM is 2 meters, a clamp filter with a value of 15,10 represents a filter of 15 meters by 15 meters is applied, and no pixel can be changed by more than 2 meters times 0.1, which equals 0.2 meters (or 20 centimeters). A clamp filter can be useful making features, such as roads, very smooth without blurring sharp slopes.

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:

All input files must be in DSM 32-bit real raster format.
All input files must be in PCIDSK format (.pix).

Module details

The reads the input folder for DEM images and applies terrain filters to generate a high-resolution DTM. The module automatically converts the DSM to a bare-Earth DTM by applying a series of filtering steps to remove features, such as buildings and trees. Small bumps and pits are also removed, and a median filter is applied to produce a smooth result.

The input must have a minimum of one channel: however, if the data has a channel that contains a DEM, that channel is used. If more than one channel contains a DEM, the last one is used. If no channel contains a DEM, the last channel is used.

Tip: Occasionally, you may need to touch up areas of difficulty, such as removing large buildings, in the DTM in the DEM Editing window of Focus.

Job results

On successful completion of processing, the following files are created:

For each input file, a processed DTM file in PCIDSK format (.pix).
The naming convention of the output file or files is based on that of the input.