Add Trend Line

Use the Grid and Image > Gridding > Trend Enforcement > Add Trend Line menu option (TRNDDIGI GX) to add your own trend line to a database.

Your trend line can be included in the gridding process to enforce a particular local trend in the data. To use this GX you should have a database and a map open in the project. The database should contain the original data that is gridded and the map should display a grid image of the data (which may already be trended) together with the current line or data point locations.

Add Trend Line dialog options

Grid to digitise	Specify the name of a grid file from which to extract data values at the defined trend points. This grid should have been created from the original data in the database, or a trended version of the original data. The grid should not be filtered and the grid cell size should be small enough to sample all information of interest.
Output channel	Select the channel in which to place the digitised values from the grid. This should be the same data channel that is/will be gridded.
Re-sample interval	Specify an interval at which to resample the data along the defined trend line. We recommend that the trend line be re-sampled at the cell size of the final grid. If this field is left blank, the trend line will not be re-sampled. When a trend line is re-sampled, only the digitised points are used (X, Y and Z value from the grid), and a smooth line is interpolated for both position and Z value at the resample interval. If the data will be gridded using BIGRID (line gridding), re-sampling is not required (but still recommended) since BIGRID will automatically resample along the trend lines as part of the gridding process. If gridding using minimum curvature gridding (RANGRID) or Kriging (KRIGRID) or any other gridding process for randomly located data, you MUST specify a re-sample interval. This will create data along the trend line that will then be used as part of the gridding process.
OK	After you press the OK button, you will be prompted to digitise the required trend lined from the current map. See the application notes below for more information on this process. Click the right mouse button and select Done when finished. If digitizing trend lines for BIGRID (line gridding) trend lines cannot change direction relative to the gridding direction or they will be rejected from the gridding process. If you do need to create a trend line that changes direction, digitise the trend line in separate pieces.

Application Notes

When digitizing the trend line, you should select points either exactly on current survey lines or located on existing data locations. The grid values at these points will match the original data very closely unless you have applied a filter to the original data. Also, the grid being digitised should not have a grid cell size that is too large relative to the sample interval of the data.

The digitised trend line will be interpolated to the specified re-sample interval (if specified) and added to then end of trend line "R2" in the database. Individual trend lines will be separated from the previous line by a dummy value in the "R2" data line. Trend lines "R0" and "R1" are normally created by the automated trending algorithm used by the TRENDDB GX.

Run the GX repeatedly to create more trend lines.

To edit a trend line, select the Shadow cursors button in the map toolbar (the button with the cross-hair) and display line "R2" in the database showing the "X", "Y" and data channel. You can then click on manual trend line points to see their location in the database and make any adjustments as required. Normally, you will just delete entire trend lines and re-create new lines as required. To delete a trend line, select all the trend line points in the "fiducial" column of the database display (the left-most column), right-mouse and select Delete marked rows/fids.

When a data set is gridding that includes trend lines, the trend lines will force the gridding process to honour the trended data.

Note that trend lines must be selected to be included in the gridding. You may want to de-select the trend lines for other data processes with the database.