DEMADJUST

Description

Adjusts (raises/lowers) a raster DEM to elevation points held in a vector layer so that it better fits the elevation points. Typically the points are surveyed elevations or are ground control points (GCPs) and the raster DEM is one that has been generated from airphoto or satellite stereo imagery.

Parameters

Name	Type	Length	Value range
InputVec: Adjustment elevation points *	Vector port	1 - 16
InputDEM: Elevation layer *	Raster port	1 -
Output: Adjusted DEM *	Raster port	1 - 1
Input vector elevation field	String	0 - 1	ZCOORD | Field Name Default: ZCOORD
Adjustment method	String	0 - 1	All| Under | Clamp | Under Sigma | Clamp Sigma Default: All
Adjustment value	Float	0 - 1
Use average	Boolean	0 - 1	TRUE | FALSE Default: FALSE

* Required parameter

Parameter descriptions

InputVec: Adjustment elevation points

Specifies the input vector segment(s) containing points representing known elevations.

InputDEM: Elevation layer

Specifies the raster DEM to adjust.

The channel must have the metadata tag ELEVATION_DATUM set either to mean sea level (MSL) or ellipsoidal (ELL) value.

Output: Adjusted DEM

The adjusted DEM raster.

Input vector elevation field

Specifies the field containing the elevation (or Z) values. The field name must exist in the attribute table of the vector layer(s).

Adjustment method

Optionally specifies the adjustment algorithm that should be used. This, along with the Adjustment value and Use average parameters, controls which points should be used and if there should be any limits on magnitude of the adjustment that will be applied.

Adjustment value

The value to be applied to the selected Adjustment method. If the selected method is Under or Clamp, this is an absolute value. If the selected method is Under Sigma or Clamp Sigma, this is a sigma value (i.e., the square root of the variance). For the All option, this parameter is inactive.

Use average

Choose whether to use the average difference for the selected Adjustment method. Each of the methods can use an average value in which case the entire DEM is raised (or lowered) by a single value (the average difference) rather than trying to create a varying surface that goes through the points. This is useful in removing a systematic error.

Details

DEMADJUST adjusts (raises/lowers) a raster DEM to elevation points held in a vector layer. Typically the points are surveyed elevations or are ground control points (GCPs) and the raster DEM is one that has been generated from airphoto or satellite stereo imagery, or from SAR interferometry. Altering the raster DEM in this manner can help reduce errors in the DEM. Depending on the options used it is possible to adjust the DEM to exactly fit every point, closely match points within certain parameters, or use an average of the differences to remove a single systematic error.

DEMADJUST supports geocoded stereo DEMs and interferometric SAR DEMs in the raw (slant range) geometry. The interferometric DEMs are automatically identified by the presence of the binary math model in the input DEM file.

If adjustment is being applied to DEMs generated from stereo imagery it is HIGHLY recommended that this method only be used on raster digital terrain model (DTM) rather than the digital surface model (DSM). The DSM is the initial product produced by automatic DEM extraction and the DTM is produced either automatically (e.g., via DSM to DTM filtering) or via manual editing. In addition it is suggested that the 'flattening' of lakes and other water bodies by done after the DEM adjustment since the adjustment step will modify the entire DEM surface and large flat areas are unlikely to remain completely flat.

The vector segment/layer(s) specified in DBVS are used for the elevation points. Only points will be used - lines and polygons are ignored. To utilize lines, points and polygons in adjusting DEMs in localized regions the function RVDEMINT should be considered. Points outside the DEM area or in no data areas are ignored. The elevation is typically taken from the Z value of the points (Input vector elevation field set to "ZCOORD" or FLDNME="ZCOORD" or FLDNME="") however this can be changed by specifying the name of a numeric attribute field.

The points will automatically be reprojected to match the projection of the raster DEM. The module will terminate execution if it cannot project between the vector and DEM coordinate systems. It is the user responsibility to ensure that the raster DEM and vector points are in the same elevation units (feet or meters) and the input DEM has the ELEVATION_DATUM metadata tag set to an MSL or ELL value. The code checks for the same tag in vector layer(s); if it is not found, point elevations are assumed to be relative to the mean sea level (MSL). If the vector and DEM datums are different, the module will automatically convert points to the DEM channel elevation datum. If the datum conversion is applied, the adjusted DEM elevations at vector point locations may be different from the elevations of the points themselves.

The vector points are compared to the raster to find the difference between them. These differences are used to interpolate a 'difference surface' over the ENTIRE raster DEM, regardless of how far apart the points are. For example, if there was a single point then the entire raster DEM would be raised or lowered by the difference. This can be a dangerous operation if a point is over an error in the raster DEM (for example, the elevation point is on the top of a hill which has been mistakenly removed in the raster DEM, in which case the adjusted DEM will have a very significant error over a large area). To prevent this it is possible to limit the amount of adjustment using the Adjustment method (ADJMETHOD) parameter.