SLASP

Description

Calculates the surface slope and aspect angles for each pixel of an elevation channel. Output can be written to an existing file or to a new file. When values are written to a new file, the slope channel is created first, followed by the aspect channel. The values in the slope channel range between 0 and 90 degrees, and are used to describe the angle of incline or decline at a given pixel. The values in the aspect channel describe the direction in which the slopes are facing. These orientation angles range from 0 to 360 degrees, where 0 degrees represents the top (usually North) and 90 degrees represents the right side (usually East). Pixels without slope (0 degrees) are assigned a special "no aspect" value.

Parameters

Name	Type	Length	Value range
InputDEM: Input elevation channel	Raster port	0 - 1	Default: 1
OutputSLP: Output slope channel	Raster port	0 - 1
OutputASP: Output aspect channel	Raster port	0 - 1
Elevation units	String	0 - 1	METER | FEET | US_FEET
Elevation scale	Float	0 - 1	Default: 1.0
Elevation offset	Float	0 - 1	Default: 0.0
Background elevation value	Float	0 - 1
Zero slope aspect value	Float	0 - 1	Default: 510.0

Parameter descriptions

InputDEM: Input elevation channel

Specifies the input channel that contains the elevation data.

OutputSLP: Output slope channel

Specifies the output channel to receive the slope data.

The channel receiving the slope data can be 8-, 16- or 32-bit, although a 32-bit channel is recommended to preserve precision.

The output channel cannot be the same as the input channel.

OutputASP: Output aspect channel

Specifies the output channel to receive the aspect data.

The channel receiving the aspect data must be either 16- or 32-bit. If an 8-bit channel is specified, SLASP will error.

The output channel cannot be the same as the input channel.

Elevation units

Specifies the units used to describe the elevation values of the input DEM file (FILEDEM).

This parameter is used to ensure that the vertical (elevation) unit matches the horizontal (projection) unit. If a discrepancy exists, the elevation unit will be converted to the projection unit to calculate slope and aspect. If the projection is lat/long, an on-the-fly conversion from degrees to the unit specified by this parameter is performed before combining them with the elevation values for computing slope and aspect.

If this parameter is not specified, the program checks for an ELEVATION_UNITS metadata tag at the file level, and again at the channel level.

If the elevation unit is not specified or is not found in the metadata, it defaults to METER.

Elevation scale

Specifies the values used to scale the DEM pixel values to values in the units indicated by the Elevation Units parameter.

The conversion formula is:

elevation_value = scale * (DEM_pixel_value + offset)

If this parameter is not specified, SLASP checks for an ELEVATION_SCALE metadata tag at the file level, and again at the channel level.

If this value is not specified or found in the metadata, the scale defaults to 1.0, indicating that the scale is 1:1.

Elevation offset

Specifies the values used to shift the DEM pixel values to values in the units indicated by the Elevation Units parameter.

The conversion formula is:

elevation_value = scale * (DEM_pixel_value + offset)

If this parameter is not specified, SLASP checks for an ELEVATION_OFFSET metadata tag at the file level, and again at the channel level.

If this value is not specified or found in the metadata, the offset defaults to 0.0, indicating that there is no offset.

Background elevation value

Specifies a special value, in the input elevation channel, used to indicate which pixel value is to be handled as no data (no elevation)

If this parameter is not specified, the program checks for a NO_DATA_VALUE metadata tag at the file level, and again at the channel level.

If this value is not specified or found in the metadata, all pixels of the DEM are assumed to be valid.

Zero slope aspect value

Specifies the digital value assigned to pixels in the output aspect map, where slope is 0 (No slope). If this parameter is not specified, these pixels are assigned a value of 510.

This parameter is used exclusively for the aspect map.

Details

SLASP uses elevation values held in a DEM to calculate the corresponding slope and aspect angles. The slope algorithm used in this function is pixel-based, meaning that there is no second- or third-order polynomial surface fitting process. This also means that the slope and aspect maps may uncover some existing artifacts from the elevation channel. The algorithm is based on the paper cited in References.

Accurate computation of the slope and aspect values depends on the proper specification of the elevation unit (ELEVUNIT), as well as the elevation scale and offset (ELFACTOR). Specifying the elevation unit (ELEVUNIT) allows SLASP to perform a preprocessing check to ensure that the projection unit and elevation unit match and, if necessary, apply an appropriate conversion. If the input DEM has been scaled, the elevation scale and offset (ELFACTOR) parameters should be specified to define how to properly un-scale the elevation values prior to computing slope and aspect.

When a DEM with a lat/long projection is provided, SLASP converts the resolution of a pixel from degrees to the specified elevation unit (ELEVUNIT). This conversion is used solely for the correct computation of slope and aspect and will not affect the georeferencing of the output file.

References

Corripio, J.; Vectorial algebra algorithms for calculating terrain parameters from DEMs and solar radiation modeling in mountainous terrain. International Journal of Geographical Information Science, 2003, 17 (1), 1-23.