Load Corrections
Use the Load Corrections menu option (XLEVEL GX) to create level information channels by extracting required data from an intersection table. An intersection table can be created using the Intersections menu option.
Make a Level Channel dialog options
Application Notes
XLEVEL, INTERSCT and LEVEL can be used to apply tie-line levelling to time varying data such as a total magnetic field survey. Tie-line levelling is a technique used to adjust the data along each survey line so that survey lines and tie lines will have the same values where they intersect. In this description we assume that we are dealing with magnetic data, but the same procedure can be followed for any type of data that has time variations.
If magnetic ground base station data is available, the data to be levelled should first be corrected for the recorded time variation using the MAGBASE GX . Ideally, after base station corrections have been applied, the survey data should not require any further levelling. However, the magnetic field does not always vary uniformly over the distances involved in a large magnetic survey, and tie line levelling may still be required.
Tie line levelling basically involves two steps. First, tie lines are levelled by assuming that the average difference between a tie line and all crossing survey lines represents a simple base level correction. This assumes that there are a sufficient number of crossing lines to represent a statistical average of the time varying error along each tie line. This further assumes that the tie line has a base level shift only. Second, all survey lines are adjusted to match the tie lines values at their intersection.
This procedure involves the following steps:
1. Run INTERSCT to create a tie line intersection table.
2. Run XLEVEL to extract level corrections, level errors and line cross gradients along each tie line.
3. Manually inspect each tie line in comparison with the intersection data and remove suspicious intersections.
4. Run STATLEV to adjust each tie line to a statistical trend of the tie errors.
5. Run INTERSCT to create a new intersection table using the levelled tie lines.
6. Run XLEVEL to extract level corrections, level errors and tie line cross gradients along each survey line.
7. Manually inspect each line in comparison with the intersection data and remove suspicious intersections.
8. Run FULLLEV to level the survey lines to the tie lines.
The resulting lines will be levelled to match the survey lines exactly at their intersection points.
The above methodology, including steps 1-7, is automated using the "Iterative Levelling" GX method.
Intersection Databases
The Geosoft database can be used to store the intersections information. This has several benefits:
1. The exact intersections locations can be plotted to a map using the normal mapping and symbol methods.
2. Quantities such as the mis-tie or gradient values can be easily gridded for display.
3. Intersections are sorted by Tie Line, instead of appearing as a single long list.
To create an intersections database, simply run the Levelling Intersections GX and specify a file of type (*.GDB) as the output intersection table. The XLEVEL GX recognizes both the original *.tbl file format and the new Geosoft Database file as inputs. Each Tie in the intersection table gets its own line in the Geosoft Database file, containing all the line intersections with that tie line.
To create an intersection database, simply run the INTERSCT.GX and specify a file of type (*.GDB) as the output intersection table. The XLEVEL GX now recognizes both the original *.tbl file format and the new Geosoft Database file as inputs. Each Tie in the intersection table gets its own line in the Geosoft database file, containing all the line intersections with that tie line.
The channels of the intersection database:
X
X coordinate of the intersection
Y
Y coordinate of the intersection
TIE
Tie line number
TIE_FID
Tie line fiducial
TIE_LEVEL
Tie line reading
TIE_GRAD
Tie line gradient (per fiducial)
LINE
Line number
LINE_FID
Line fiducial
LINE_LEVEL
Line reading
LINE_GRAD
Line gradient (per fiducial)
CROSS_DIFF
Miss-tie value (LINE_LEVEL - TIE_LEVEL). Note that when the XLEVEL GX is used to extract mis-tie values to the flight database using the "Level Lines" option, the resulting CROSS_DIFF values in the flight database are of the opposite sign, i.e. TIE_LEVEL - LINE_LEVEL.
MASK
The MASK channel is used to select/deselect individual intersections. Any row of data where the mask channel is set to a dummy is excluded when the XLEVEL GX is called. Its operation is completely independent from the use of the maximum gradient parameter (see above).
See Also:
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