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

Intersection table	Input intersection table or database, usually created by the INTERSCT GX. See more on using intersection databases below under "Intersection Databases". Script Parameter: XLEVEL.INTERSECT
Maximum gradient (Z/fid)	If this value is defined, then only intersections with both line and tie-line gradients less that this limit will be considered. Normally, you should include all intersections until you have gained enough experience with the data and survey system to establish reasonable limits. If a Geosoft Database intersection table is used, intersections are selected using both the maximum gradient and the mask channel parameters, so intersections having either a gradient above the maximum, or a dummy value in the mask channel, are NOT exported. Script Parameter: XLEVEL.MAXGRADIENT
Process line types	"LINE" to extract corrections to flight lines. "TIE" to extract the corrections to tie lines. "SELECTED" to extract only to selected lines. To level tie-lines, select "TIE". Script Parameter: XLEVEL.TYPE
Unlevelled data channel	The original data channel to be used as a reference. The fiducial interval of this channel will be used as the fiducial base for the extracted leveling channels. Script Parameter: XLEVEL.REF
cross-level channel	The optional output channel in which to place the cross level values. These are the values of the crossing lines at the intersections with the line being viewed. The level channel is not required, but it can be useful to view the tie line levels graphically in profile when determining the acceptance of each intersection. Script Parameter: XLEVEL.LEVEL
Output difference channel	The output channel in which to place the difference between the survey lines and tie lines. The level difference channel will be used to correct the tie lines by statistical leveling Script Parameter: XLEVEL.ERROR
Output cross-gradient channel	The optional output channel in which to place the gradient of the tie line at the intersection point. This channel is also not required, but this information can be used to decide if a suspicious tie point is located in a high gradient along the crossing line. Script Parameter: XLEVEL.CROSS

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).

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).