Altitude Deviation

Use the Altitude Deviation option (geogxnet.dll(Geosoft.GX.AirborneQC.AltitudeDeviation;Run)*) from the Airborne QC menu to carry out quality control on survey flying height.

Altitude Deviation dialog options

Input database file

Select the database from which you wish to calculate the altitude deviations. This field is auto-populated with the name of the active database.

Vertical position defined by

Select if the vertical position is defined by the terrain clearance (vertical position from aircraft down) or by the actual elevation (aircraft elevation relative to a geoid).

Script Parameter: QCHEIGHT.VERTICAL_DEFINITION [0 – Terrain clearance (default), 1 – Elevation]

Terrain clearance channel/
Flight elevation channel

Based on the selection above, you will be prompted to specify either a terrain clearance channel or a flight elevation channel.

Script Parameter: QCHEIGHT.CHAN

Topography channel

Select the topography channel for checking the absolute altitude deviations. Required if the maximum flight elevation (see below) is specified.

Script Parameter: QCHEIGHT.TOPO

Test against

Select from one of the two options:

Terrain clearance: the tests are carried out against a constant datum (a given constant nominal aircraft height relative to the topography).
Drape altitude: the tests are carried out against a drape survey (channel).

Script Parameter: QCHEIGHT.TEST_AGAINST [0 – Terrain clearance (default), 1 – Drape altitude]

Nominal terrain clearance/
Drape elevation channel

Based on the selection above, you will be prompted to specify either a nominal flying height or to select a drape channel.

You can import the drape channel into the survey database from either a gridded drape surface or by running a lookup into the ideal planned database.

Script Parameter: QCHEIGHT.NOMINAL

Maximum deviation

Specify the maximum deviation tolerance, at each reading location, of the vertical position channel from the nominal terrain clearance or the drape elevation channel.

See the Application Notes below for more details.

Script Parameter: QCHEIGHT.MAXDEV

Allowed deviation over a distance

Allowed deviation

Specify the allowed deviation of the vertical position channel from the nominal terrain clearance or the drape elevation channel over a specified distance (see below).

Script Parameter: QCHEIGHT.ALLDEV

Over a distance of

Input the distance over which the altitude deviation tests are carried out.

See the Application Notes bellow for details on the specification tests and results.

Script Parameter: QCHEIGHT.DIST

At least one of the two specifications, maximum deviation or allowed deviation over a distance, must be defined.

Minimum terrain clearance

If specified, this is an absolute minimum height. All points below this height relative to the topography are flagged.

Script Parameter: QCHEIGHT.MINHEIGHT

Maximum flight elevation

If specified, this is an absolute maximum height. All points above this height relative to the topography are flagged.

Script Parameter: QCHEIGHT.MAXELEVATION

Plot results

Specify if the outcome (flight lines, flagged locations, legend) should be plotted on the current map, on a new map, or not at all:

Existing map: this is the default option. If the legend group already exists on the map ('LEGEND_QC_<database>' in the Base view), new legend items will be added to it. If this group already exists and contains a survey path ('PATH_<database>' group in the Data view), the path will not be replotted, and the Line Attributes tab in the expanded section will not be active. The Flag Attributes tab will allow you to customize the colour, thickness, and offset from the survey path of the flagged locations.
New map: the Line Attributes tab in the expanded section is active and prompts for the label size and position as well as the colour, thickness, and break on gaps of the survey path.
Do not plot: the output plot will not be generated. The two Attributes tabs are hidden.

In order to visualize multiple QC test results on the same map, flag symbols signifying data that are outside of specifications are plotted using a unique colour and offset for each QC test.

Script Parameter: QCHEIGHT.MAP [0 – Display on current map (default), 1 – Display on a new map, 2 – Do not plot]

Map name

Specify the name of a new or existing map on which to plot the results. The map will open in the foreground. If the map is new, you will be prompted to interactively specify the location of the legend. New information will be added to the map; the legend will indicate the line path colour as well as the colour of the flagged locations and the percentage of the survey that is not within the specifications.

When a new map is generated, the map is scaled to the input database extents, and the map projection is set to the projection of the database.

Script Parameter: QCHEIGHT.MAP_FILE

[More]

Flag Attributes

Colour

Readings exceeding the specified tolerance are indicated by colour-coded symbols along the survey path. Select a colour for plotting these symbols. By default, this is set to light blue.

Script Parameter: QCHEIGHT.FLAG_COLOR

Thickness

Specify the symbol size in mm. By default, it is set to 0.2.

Script Parameter: QCHEIGHT.FLAG_THICKNESS

Offset

Specify the symbol offset (the distance from the survey line) in mm. By default, it is set to 0.4.

A negative value will plot the symbols to the left of the survey line; a positive value will plot the symbols to the right of the survey line. Set the offset to 0 to plot the symbols on top-of-the-line path.

Script Parameter: QCHEIGHT.FLAG_OFFSET

Line Attributes

Colour	Select a colour for plotting the line path. By default, the colour is set to black. Script Parameter: QCHEIGHT.LINE_COLOR
Thickness	Specify the line thickness in mm. By default, it is set to 0.15. Script Parameter: QCHEIGHT.LINE_THICKNESS
Label location	Select the label position relative to the ends of the lines: End (default): the labels are displayed centered off the ends of the line. Above: the labels are displayed above the ends of the lines. Below: the labels are displayed below the ends of the lines. None: the line labels are not plotted. Script Parameter: QCHEIGHT.LINE_LABEL_LOCATION
Label size	Specify the label size in mm. The default is 2.5. Script Parameter: QCHEIGHT.LINE_LABEL_SIZE
Break on gaps>	Specify the maximum gap (in units of distance) before breaking the line. The default is 0 (no gap). Script Parameter: QCHEIGHT.LINE_BREAK_ON_GAPS

Application Notes

This GX assumes that the current database is the one containing the actual survey data. The database is called the survey database. If you wish to process a different database, you may browse to add it to the project or select one from the workspace list.

Quality Control Specifications and Indicators

This GX permits four separate specifications to be tested to determine whether the airplane flight heights are within specification or not:

1)	At each reading location, the flight height must be less than the nominal flight height plus the maximum deviation and more than the nominal flight height minus the maximum deviation.
2)	The flight height must be less than the nominal height plus the allowed deviation or more than the nominal height minus the allowed deviation for a distance less than the specified distance.
3)	The terrain clearance must be above the minimum terrain clearance at all times.
4)	The flight height must be below the maximum elevation at all times.

You must specify either the maximum deviation or the allowed deviation. The check against the absolute flying height range takes precedence over the Maximum deviation and Allowed deviation tests. To disable a test, leave the corresponding field(s) empty.

This GX creates a flag channel, Flag Height, which contains numbers from -4 to 4 inclusive to indicate the result of the quality control test(s) on flight line height.

Indicator Definition

Flag	Description
0	Flight height passes both tests.
±1	Flight height fails the 1^st test (maximum deviation).
±2	Flight height fails the 2^nd test (allowed deviation over a distance).
±3	Flight height fails both tests.
±4	Flight height is above maximum elevation (+) / below minimum (-) terrain separation.

Positive values in the flag channel indicate that the actual flight height is higher than the nominal height, and negative values indicate that the actual flight height is less than the nominal height.

Distance Calculation

When the Allowed deviation over distance specification is tested, the cumulative distance along the actual survey path is used instead of the distance along the ideal line path.

*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 Run GX for more details on running a GX.NET interactively.