Velocity Deviation

Use the Velocity Deviation option (geogxnet.dll(Geosoft.GX.AirborneQC.VelocityDeviation;Run)*) from the Airborne QC menu to calculate the speed of airborne data collection and to carry out quality control on the calculated survey aircraft velocity.

Velocity Deviation dialog options

Input database file

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

Velocity channel

This is the channel to store the calculated speed. If a channel named "Velocity" already exists, the channel is automatically selected here. Otherwise, the field is pre-populated as "Velocity", and the velocity channel will be generated.

See the Application Notes below for details on velocity channel calculation.

Script Parameter: QCVELOCITY.CHAN

Time channel

Select the time channel. This field is enabled and mandatory if the velocity channel will be calculated.

Time channel values must be stored in decimal hours.

Script Parameter: QCVELOCITY.TIME

Nominal velocity

Specify the nominal velocity (Km/hr) against which the maximum deviation and allowed deviation options are applied.

Script Parameter: QCVELOCITY.NOMINAL

Maximum deviation

Specify the maximum deviation from the nominal velocity. If there are any instances where the velocity is outside the nominal velocity plus or minus the maximum deviation, the data will be flagged as exceeding the specifications.

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

Script Parameter: QCVELOCITY.MAXDEV

Allowed deviation over a distance

Allowed deviation

Specify an additional deviation tolerance from the nominal velocity for a given distance (see below).

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

Script Parameter: QCVELOCITY.ALLDEV

Over a distance of

The distance over which the velocity deviation tests are carried out. This means that the survey speed along the survey line should not exceed the nominal velocity plus or minus the allowed deviation for more than the specified distance here.

Script Parameter: QCVELOCITY.DIST

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

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: QCVELOCITY.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: QCVELOCITY.MAP_FILE

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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 purple.

Script Parameter: QCVELOCITY.FLAG_COLOR

Thickness

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

Script Parameter: QCVELOCITY.FLAG_THICKNESS

Offset

Specify the symbol offset (the distance from the survey line) in mm. By default, it is set to 0.3.

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: QCVELOCITY.FLAG_OFFSET

Line Attributes

Colour	Select a colour for plotting the survey path. By default, the colour is set to black. Script Parameter: QCVELOCITY.LINE_COLOR
Thickness	Specify the line thickness in mm. By default, it is set to 0.15. Script Parameter: QCVELOCITY.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: QCVELOCITY.LINE_LABEL_LOCATION
Label size	Specify the label size in mm. The default is 2.5. Script Parameter: QCVELOCITY.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: QCVELOCITY.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.

Velocity Channel Calculation

The coordinate (X & Y) channel units are used to convert the output velocity as _Distance/dt in units of km/hr. If the "_Distance" channel does not exist, it will be generated and calculated from the X & Y channels.

The _Distance channel is always recalculated even if the velocity channel already exists.

The velocity is calculated at a given row using the values before and after, and the final values are interpolated and extrapolated ("nearest" at the ends) to fill in dummies.

If an existing velocity channel is selected, all the input values are assumed to be in the same units. The km/hr units are only enforced if the velocity channel is new (and calculated using the X & Y and _Distance channels); then the input deviation values are also assumed to be in km/hr.

Quality Control Specifications and Indicators

This GX allows two separate specifications to be applied to check whether the velocity is within specification:

1)	At each reading location, the velocity should be no less than Nominal velocity - Maximum deviation, and no more than Nominal velocity + Maximum deviation.
2)	At each reading location, the velocity should be no less than Nominal velocity - Allowed deviation, and no more than Nominal velocity + Allowed deviation over a distance greater than that specified.

You can apply one or both of the specification tests: maximum deviation and allowed deviation over a distance. To disable a test, leave the corresponding field(s) empty.

The GX creates a flag (mask) channel called Flag_Velocity, which contains numbers from -3 to +3 inclusive to indicate the results of the quality control test(s) and how the actual velocity values deviate from the ideal velocity.

Indicator Definition

Flag	Description
0	Velocity = Nominal velocity +/- Maximum deviation. Velocity passes the test(s).
+1	Velocity > Nominal velocity + Maximum deviation but Velocity < Nominal velocity + Allowed deviation over specified range
-1	Velocity < Nominal velocity - Maximum deviation but Velocity > Nominal velocity - Allowed deviation over specified range
+2	Velocity > Nominal velocity + Maximum deviation but Velocity < Nominal velocity + Allowed deviation exceeding specified range
-2	Velocity < Nominal velocity - Maximum deviation but Velocity > Nominal velocity - Allowed deviation exceeding specified range
±3	Velocity > else. Velocity fails the test(s).

You can refer to the velocity test when you encounter data outside the acceptable threshold in other tests and identify if the poor quality is due to variations in the speed of the data collection. Based on how the data is collected, the reported velocity at each measurement can be a very good indicator of the performance during data acquisition.

Plotting the Results

The results are plotted on the map using a single colour. The symbols for all the flagged locations (Flag_Velocity !=0) are plotted in ground units to the left or to the right of the line path based on the specified offset.

If you would like to illustrate the type of failure on the map, generate a symbol plot of the Flag_Velocity channel using any one of the GXs: Colour Range Symbols, Proportional Size, and Colour Range.

*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.