4.3. Processing terrain corrections

4.3. Processing terrain corrections
	Chapter 4. Work with lines and stations

Current version of the BlueWhaleProcessing provides partial support for semi-automatic terrain corrections computation. The following tasks are supported:

Automatic computation of regional terrain correction with the use of SRTM3 digital terrain model SRTM3 data with both Hammer method and Plouff method.
Support for user-provided terrain data to be included into regional corrections computation in addition to the stations elevations data and SRTM3 digital terrain data (Hammer method only).
Simple tool for estimation of difference between SRTM3 elevation datum and stations database elevation reference.
Indirect support of manual combination of regional terrain corrections computed by the BlueWhaleProcessing software and local near zone corrections.

	Warning
	Terrain corrections utilities are under development and improvements in both user interface and data structures can be expected in the future versions of the BlueWhaleProcessing software.

4.3.1. Regional terrain corrections

BlueWhaleProcessing software currently provides two methods for computation of regional terrain corrections. Both methods use worldwide SRTM3 digital terrain model with 3 arcsec (~90 m) spacing.

Hammer method [Hammer 1939]

Hammer corrections are computed for zones D-M, i.e. for distances 53 - 21943 m from the station in concentric rings divided into segments. Elevation difference of each segment from the station is computed as mean difference of the station elevation from all SRTM cells whose center falls into the segment.

In addition to the SRTM3 data, all stations elevations are included into computation, i.e. each existing station in the project contributes to the terrain corrections calculation.

Finally, user-provided terrain data can be imported into the project.

Plouff method [Plouff 1976]

Plouff method models terrain influence with series of vertical prisms. Each SRTM3 cell represents a single prism. Terrain correction effect is computed for each SRTM3 cell with it's center between 80 m and 120 km from the station. If SRTM3 data are missing for a particular cell, mean value from the neighbour cells is used.

In normal project lifecycle, computation of terrain corrections takes place as the final processing step. BlueWhaleProcessing software allows computation of terrain corrections in any stage of processing to provide full featured intermediate snapshot data, nevertheless it is recommended to clear all terrain corrections and recompute the whole project terrain corrections from scratch when all project data are collected.

To compute regional terrain corrections:

Select Tools - Options from the main menu. Select location where downloaded SRTM3 data will be saved. If local mirror of the official SRTM server is available, you can modify the default download address accordingly. HTTP protocol only is supported.
The settings in the Options dialog will affect each project in your BlueWhaleProcessing installation.

In Project properties Gravity processing tab click the Compute difference by the SRTM difference field.

The software will close the Project Properties dialog and recompute the entire project. As a part of recomputing, needed SRTM3 data are downloaded. For each station, elevation from the SRTM3 terrain model is interpolated and the difference between the station elevation and the interpolation is computed. The progress bar displays intermediate results.

When recomputation is finished, SRTM Difference field value in the Project Properties is replaced by the result of the computation. Revise and correct the value if needed.

Systematic difference between the SRTM3 data and precise leveling or GPS provided stations data is normal. SRTM3 model's mean absolute precision is ~ +/- 16 meters and in some regions the difference can be even higher. Relative precision is much better and as soon as the data are corrected for the systematic absolute shift, terrain corrections computation provides satisfactory results in most cases.

	Tip
	You can plot SRTM difference map, i.e. map of differences between the interpolated SRTM model elevation and station's elevation to assess overall fit of the SRTM data over the project area and to discover sources of possible discrepancies. See Creating maps chapter for more information about creating maps.

In Project properties Gravity processing tab select method for terrain corrections computation from the drop-down menu:

Hammer

Hammer method will be used for terrain correction computations.

Prism (Plouff)

Plouff method will be used.

Manual

Terrain corrections will not be computed automatically by the BlueWhaleProcessing software. You can enter terrain corrections manually for each station in the Stations editor window.

	Tip
	When manual method is selected, terrain corrections can be imported with stations coordinates data. See the Import stations coordinates section for more information.

None

All terrain corrections in the project will be cleared with the next project recomputation.

Close and reopen the project. Necessary SRTM3 data will be downloaded and terrain corrections will be computed for stations where terrain corrections were not computed yet.

Note

Changing of the computation method will not recompute existing terrain corrections. Terrain correction of a station will be recomputed automatically with the next project recalculation only if station coordinates have been changed.

To recompute the entire project, select terrain corrections computation method to None first. Then close and reopen the project. All terrain corrections will be cleared. Finally, change the terrain corrections settings to the new value and close and reopen the project again.

Computation of terrain corrections needs reasonable amount of time. Plouff method is significantly slower than Hammer method.

4.3.2. User-provided terrain data import and processing

When Hammer method is used for terrain corrections computation, user-provided terrain data can be imported into the project and used for computation.

To import terrain data:

Prepare the terrain data file in CSV format. Structure of the file is the same as the structure of stations coordinates file.
Right-click the project in the Projects window and select Import terrain data from the context menu.
Select file with your terrain data. Finally, completely recompute all terrain corrections as described above.

	Note
	In order to remove imported terrain data from your project, right-click a project containing imported terrain data and select Delete terrain data from the context menu. Recompute all terrain corrections afterwards.

4.3.3. Adding local near-zone corrections into the project

Current version of the BlueWhaleProcessing software does not contain direct support for computation of local near-zone (Hammer zone A-C) terrain corrections which are normally based on operator's estimates at each station, nor support for combination (superposition) of local and regional terrain corrections.

As a workaround, a spreadsheet program, e.g. Microsoft Excel or OpenOffice Calc can be used for computation of local corrections and combination of import/export functions of the BlueWhaleProcessing software can help combine regional corrections computed by BlueWhaleProcessing and local corrections from the spreadsheet.

To combine regional corrections from the BlueWhaleProcessing project with local terrain corrections:

Export entire project data into CSV format. Open the CSV file in a spreadsheet program.
Rename the Terrain corr. field e.g. to Regional corr. Add a new column to the sheet containing your computed local terrain corrections and another new column with the sum of regional and local corrections. Name the later column Terrain corr. Save the spreadsheet in the CSV format.
In the BlueWhaleProcessing project switch method of terrain corrections computation to Manual.
Import the updated CSV file into your project. Regional terrain corrections computed by the software will be replaced with the combined corrections.