Intersection Levelling
Use the Intersection Levelling option (geogxnet.dll(Geosoft.GX.UAVSurvey.IntersectionLevelling;Run)*) from the Level menu to perform tie line levelling on the UAV survey data.
Intersection Levelling dialog options
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Input data channel
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Select the data channel to perform tie line leveling on.
Script Parameter: PLAN_UAV_SURVEY.INPUT_CHANNEL
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Intersection tolerance
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Specify the intersection tolerance in distance units. If set to 0.0, points on the survey and tie line must coincide to be identified as an intersection. If set to > 0.0, the points of closest approach to the intersection that are within the tolerance will be used. An intersection table is created that is subsequently used.
Script Parameter: PLAN_UAV_SURVEY.INTERSECTION_TOLERANCE
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Maximum gradient per fiducial
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Specify the maximum gradient in data units per fiducial. If this value is defined, then only intersections with both line and tie-line gradients less that this limit will be added to the intersection table. Initially, it is recommended to include all intersections by providing a large maximum gradient value and observe how well the data is levelled. You can then opt to refine this parameter and establish a reasonable limit according to the outcome.
Script Parameter: PLAN_UAV_SURVEY.MAX_GRADIENT_PER_FID
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Recalculate intersections
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An intersection table is created using the above 3 parameters and saved in the working folder. The name of this table is _TempOutputIntersection.tbl. This table is used if the GX Intersection Levelling is invoked again and the above parameters are ignored. To recalculate the intersection table with new parameters, check this option and run Intersection Levelling again.
Script Parameter: PLAN_UAV_SURVEY.RECALCULATE_INTERSECTIONS ( 0 – No, 1 –Yes)
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Output levelled channel
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Specify the output levelled channel.
Script Parameter: PLAN_UAV_SURVEY.OUTPUT_LEVELLED_CHANNEL
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[More]
All subsequent parameters have default settings. Should you wish to customize them expand this section.
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Cross-level channel
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The optional output channel in which to place the final cross level values, after the levelling is completed. 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: PLAN_UAV_SURVEY.CROSS_LEVEL_CHANNEL
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Difference channel
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Upon completion of the levelling, the difference between the survey lines and tie lines is written to this channel. It is also used during the levelling process to store the current mis-tie values.
Script Parameter: PLAN_UAV_SURVEY.DIFFERENCE_CHANNEL
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Cross-gradient channel
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The optional output channel in which to place the gradient of the tie line at the intersection point. These are the final gradient values, after the levelling has been completed. This channel is also not required, but the information can be used to decide if a suspicious tie point is located in a high gradient along the crossing line.
Script Parameter: PLAN_UAV_SURVEY.CROSS_GRADIENT_CHANNEL
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Trend order
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Order of difference trend polynomial. Use a 0 order to adjust the tie lines to the average of the crossing line differences. If you can clearly see a trend in the cross difference channel, you may consider a first or higher order trend.
Script Parameter: PLAN_UAV_SURVEY.TREND_ORDER
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Application Notes
Tie-line levelling is a technique used to adjust the data along each survey line in order to minimize the discrepancy between lines and tie lines where they intersect. By the nature of drones being able to follow the terrain very closely, the discrepancy at the intersections of a UAV survey will not be sizable. As a result this tool performs a simple levelling.
Intersection levelling involves the following steps:
- Produce a table of the intersection crossing coordinates and the discrepancy between the survey and tie line data at these crossings.
- Calculate the average difference between tie and survey lines at all identified crossings.
- DC shift the tie lines by this average.
- Adjust the survey line data to match the tie line data at the crossings.
After running the Intersection levelling, grid the levelled data to check if it is adequately leveled. If any levelling artifacts are still present in the data, run the intersection leveling process on the leveled channel.
*The GX.NET tools are embedded in the geogxnet.dll file located in the "...\Geosoft\Desktop Applications \bin" folder. If running this GX interactively, bypassing the menu, first change the folder to point to the "bin" directory, then supply the GX.NET tool in the specified format. See the topic for more details on running a GX.NET interactively.
