How do I select or clip points in a TAB file that lie above or below a grid surface?

Occurrence

For example, a topographic grid has been created of an open cut pit (e.g. by overlaying a DEM grid with a 
series of donutted polygons representing pit bench levels using the Surfaces>Grid Utilities>Overlay tool).


You would like to examine those portions of the original exploration drillholes (drilled from surface) that lie
either within or below this pit surface (the drillhole downhole data tables have been converted into a series
of individual points by desurveying them (Drillholes>Calcualte 3D co-ordinates), and Creating Points
from these new XYZ co-ords).


Solution

1/. In MapInfo, create a new float -type field in the point TAB file (e.g. 'grid_value') using
Table>Maintenance>Table Structure.

2/. With both datasets open in the same map window, select all the points and run the Surfaces>Assign
Values from Grid
tool. Populate the new field (e.g. 'grid_value') with the output. Save the point table.

3/. Run a query (Query>Select) to compare the original Z value field (MZ in this example i.e. midpoint Z
of each litholigcal interval) with this new field as below. This example will return all historical drillhole
lithology intervals that are contained in the pit outline.



4/. Save this query as a table

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12th October 2007 10:39:32 am

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