Moving platform gravity import and levelling (C09)
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1. Top Library Help Top 4 Back gt INTREPID will perform the task once for each value in the Inputs list Each time it performs the task it will substitute the current Inputs value for the parameter Inputs in the task specification In this way it will process each of the splitnnnn datasets For each Inputs value the Spreadsheet Editor opens the corresponding splitnnnn dataset and creates a new field called FreeAir The Inputs list is the same as in step 1 above Repeat Begin Inputs Process Begin Name dbedit Parameters Begin Action Begin Type OpenField Name split Inputs Action End Action Begin Type CreateField Name FreeAir Dtype IEEE8ByteReal Width 10 GroupBy No Initial FA Latitude Gravity 10 Action End Parameters End Process End Repeat End The function FA is a user defined DLL SO function As used here it converts all of the values of the field Gravity to free air anomaly values which it returns as the values of the function The arguments and the returned value of the function both represent arrays of values The calculation uses the respective value of the Latitude field in each case Note The gravity value is divided by 10 before processing by FA This converts it from um s2 to mGal required by the FA function In this example the FreeAir is then converted from mGals back to um s2 by using the Spreadsheet Editor See User defined DLL SO functions in The INT2. a split dataset created from a dataset with SurveyID 5 would have line numbers 5 0001 5 0002 5 0008 Using the current version of INTREPID you can run the Split Cruise tool in batch mode or by selecting the Marine Split tool found under Levelling of the Project Manager PI SplitCruise ja File Help 4 Process splitcruise Sz nn SharpAngleTolerance 50 0 nn TrendAngleTolerance 45 0 int TrendDistanceInSamples 10 int MinimumSamplesBeforeDrop 0 nn MaximumDistanceBetweenSamples 2000 0 ay Preserve All Raw Data No e Input inputfile PD M Output outputfile DIR IEI z 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Moving platform gravity import and levelling C09 5 Library Help Top Library Help Top 4 Back gt The task specification job file splitcruise job will perform the operation for all rawnnnn datasets in a single batch task Since this task file contains a Repeat loop you must submit it to the Project Manager rather than directly to the Split Cruise tool To run the task assuming that the task specification file is called splitcruise job use the command fmanager exe batch splitcruise job INTREPID will perform the task once for each value in the Inputs list Each time it performs the task it will substitute the current Inputs value for the parameter Inputs in the task specification In this way it will create each of the sp
3. individually without having to append them together Using the current version of INTREPID you must run the Marine Level tool in batch mode The task specification job file levelpoly job will perform the Marine Level task for all split nnnn datasets in a single batch task Since this task contains a Repeat loop you must submit it to the Project Manager rather than directly to the Marine Level tool To run the task assuming that the task specification file is called levelpoly job use the command fmanager exe batch levelpoly job INTREPID will perform the task once for each value in the Inputs list Each time it performs the task it will substitute the current Inputs value for the parameter Inputs in the task specification In this way it will process each of the splitnnnn datasets For each Inputs value the Marine Level tool opens the corresponding split nnnn dataset levels the FreeAir field It stores the levelled data in a new field called FreeAir poly The Inputs list is the same as in step 1 above 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Moving platform gravity import and levelling C09 15 Library Help Top lt 4 Back gt Repeat Begin Inputs Process Begin Name marinelevel Comments Intrepid Audit Stamp ReportFile marine_level rpt Primary input dataset Dataset split Inputs InputZ splitS Inputs FreeAir OutputZ split Inputs FreeAir_po
4. job INTREPID will perform the task once for each value in the Inputs list Each time it performs the task it will substitute the current Inputs value for the parameter Inputs in the task specification In this way it will create each of the rawnnnn datasets See Special Project Manager batch task operations in INTREPID task specification job files R06 for full instructions on the techniques used in this task specification file 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Library Help Top Library Help Top Moving platform gravity import and levelling C09 4 lt 4 Back gt For each Inputs value the Import tool imports the nnnn asc file into the corresponding rawnnnn dataset Repeat Begin Inputs 0004 0053 0055 0056an etc Process Begin Name import Input Rawdata S Inputs asc Output raw Inputs Parameters Begin Format AsciiColumns AsciiColumns Begin FixedLength No SkipRecords 0 DDF marine ddf NoError No ReportDiagnostics No AsciiColumns End Parameters End Process End Repeat End Split the cruises into reasonably straight line segments using the INTREPID Split Cruise tool The Split Cruise tool creates a new SPLITLINE field which assigns a unique line number for each straight line segment This line number is a combination of the SurveyID and the sequentially numbered line segment within the dataset divided by 10000 For example
5. this import it to an INTREPID grid dataset using the Binary Image import format Use the INTREPID Subsection tool to extract the geographic region you require Use the INTREPID Spectral Domain Grid Filters to apply a low pass filter Export the resulting grid to a file in ASCII Image XYZ format 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Moving platform gravity import and levelling C09 13 Library Help Top Library Help Top 4 Back gt The exported data should be similar to the following The first line shown here is to identify the field names for these steps and is not part of the file Longitude Latitude FreeAir 118 017 6 99924 51 8000 118 083 6 99924 49 4000 118 350 6 99924 71 4000 118 383 6 99924 75 4000 118 450 6 99924 86 2000 118 483 6 99924 90 4000 118 550 6 99924 97 2000 118 583 6 99924 99 0000 118 617 6 99924 98 8000 118 750 6 99924 113 000 Sort this file in order of Longitude and save it as geosatA Sort the file in order of Latitude and save it as geosatB Append geosatB to geosatA Import geosatA into an INTREPID line dataset called geosat DIR Use ASCII Columns input format The following data definition file DDF is suitable for the example above TYPE LINE Longitude 5 11 REAL 8 IsX PROJ GEODETIC WGS84 Latitude 14 22 REAL 8 IsY PROJ GEODETIC WGS84 FreeAir REAL 4 Split the geosat DIR dataset into reasonably straight line segments using t
6. 278895 double a3 0 000023462 double sl calcgrav for i 0 i lt npts i if lat i f1t64Null amp amp obsgravity i f1t64Null sl sin lat i PI 180 0 calcgrav al 1 0 a2 sl sl a3 sl sl sl s1l print lat d 1f 1 1 1 n i lat i sl calcgrav obsgravity i FAvalue i obsgravity i calcgrav else FAvalue i 1t64Null1 return FAvalue Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Moving platform gravity import and levelling C09 8 Library Help Top Library Help Top 4 Back gt 4 Merge the separate line datasets using the Project Manager s Append operation Up until now we have processed the raw data as separate surveys one dataset per survey Whether we keep the survey data separate or merge all the surveys into a single dataset really depends upon the network adjustment levelling strategy we wish to use In this example we are going to use loop closure levelling to perform an internal network adjustment of all of the crossover points for all of the surveys Because the adjustment is completely internal ie we are using no external dataset as a reference all of the survey data must reside in a single dataset in order for the Marine Level tool to find all of the crossover points The task specification job file append_datasets job will perform the operation for all splitnnnn datasets in a single batch task Since t
7. INTREPID User Manual Moving platform gravity import and levelling C09 1 Library Help Top lt 4 Back gt Moving platform gravity import and levelling C09 wp This chapter contains a worked example showing how to level gravity data acquired on a moving platform In this example the moving platform is a ship at sea Most marine gravity data is acquired during a seismic work program The layout of the survey lines is optimised for the seismic data rather than for the gravity data As a result marine gravity data may not be acquired on a regular grid pattern In addition the user may want to level different vintages of marine gravity data to create a self consistent levelled dataset The problem is then to level a dataset which has survey lines of varying lengths and varying bearing angles The density of data observations may also vary significantly from one place to another This makes it difficult if not impossible to use standard levelling methods which have been designed for regularly flown airborne data The INTREPID moving platform marine gravity levelling was designed to apply systematic network adjustments levelling to these datasets INTREPID uses line levelling methods for this process The Split Cruise tool divides each cruise or sortie into reasonably straight line segments The Marine Level tool then applies a network adjustment to the survey data There are various options for doing this The adjustment can be
8. REPID SDK and API R18 for full instructions on creating and using these functions Note The same process is available in batch mode using the INTREPID Gravity tool 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Moving platform gravity import and levelling C09 7 Library Help Top 4 Back gt Here is a listing of the FA function This program does the IGSN71 Free Air Anomaly calculation This program uses dynamic library calls from within the INTREPID Spreadsheet Editor tool You must compile this program and put it into a shared library Under UNIX it must have the name 1ibFA so and it must reside in the directory specified by the library path SLD_LIBRARY_PATH Under Windows it must have the name 1ibFA dl1l and it must reside in a directory listed by the environment variable PATH We designed this module to be called from the INTREPID Spreadsheet Editor tool dbedit exe as FA lat obsgravity Where lat is in latitude degrees obsgravity is in mgals The prototype for a function xxx f1 f2 fn is double xxx int npts double f1 double 2 double fn include lt stdio h gt include lt math h gt double FA int npts double lat double obsgravity double PI 3 14159265358979323846 double FAvalue lat double flt64Null 5 0E 75 This represents a null value int i Perform the calculation double a1 978031 85 double a2 0 005
9. also referred to as an external reference dataset 6 Grid and examine the results Steps to follow 1 Import the ASCII files into INTREPID line datasets Follow Step 1 of the previous G o IF 23 C L example Split the cruises into reasonably straight line segments using the INTREPID Split Cruise tool Follow Step 2 of the previous example Convert the gravity field in each splitnnnn dataset from um s2 gravity to mGal free air anomaly values and store the results in a new field called FreeAir Follow Step 3 of the previous example Prepare levelling reference surface 4 Library Help Top Prepare a satellite altimeter derived gravity image for use as a levelling reference surface Essentially this involves converting an image into an INTREPID line dataset with crossover control points There are several steps required in doing this and for the purpose of this exercise we have done the preparation for you In this example we have used the Global Gravity Grid v7 2 by Sandwell and Smith These data are also referred to as the Geosat data since much of the data was derived from that satellite It has a resolution of 2 minutes approx 3700 metres or 0 033 degrees The grid was downloaded from the TOPEX FTP site FTP site ftp topex ucsd edu pub global_grav_2min The full image occupies approximately 70 Mbytes We found it beneficial to remove some high frequency noise less than 30 km wavelength from the image To do
10. ctively Note however that the Split Cruise and Marine Level tools are only available in batch mode in the current version of INTREPID Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Moving platform gravity import and levelling C09 3 Library Help Top Steps to follow 1 Library Help Top 4 Back gt Import the ASCII files into INTREPID line datasets and assign them the following names raw0004 raw0053 raw0055 raw0056an raw0096 raw0097 raw010lan raw0110an raw0127an raw0128 raw0136an raw0161 raw0167an raw0168a raw0174an raw1339 raw1359fd Use ASCII Columns input format with the following data definition file DDF TYPE LINE SurveyID 1 4 INTEGER 4 Time 6 13 REAL 8 Latitude 14 23 REAL 8 IsY PROJ GEODETIC WGS84 Longitude 25 33 REAL 8 IsX PROJ GEODETIC WGS84 Depth 34 39 REAL 4 Gravity 40 47 REAL 8 Magnetic 48 53 REAL 8 Residual 54 59 REAL 8 GROUP BY SurveyID In our example this DDF is called marine ddf Further examples of INTREPID DDFs can be found in the install_path examples DDF directory The task specification job file import job will perform the operation for all nnnn asc files in a single batch task Since this task contains a Repeat loop you must submit it to the Project Manager rather than directly to the Import tool To run the task assuming that the task specification file is called import job use the command fmanager exe batch import
11. d operation 5 Use the Marine Level tool to compute and apply a network adjustment using the method of loop closure levelling This method adjusts the data to be internally consistent and does not require an external reference dataset such as a satellite altimeter model 6 Grid and examine the results Preliminary notes The example data can be found in install_path sample_data cookbooks levelling marine The raw data for each cruise is contained in an ASCII file The files contain water depth gravity and magnetic data We are only processing the gravity data The file names are for example 0004 asc 0053 asc 0055 asc 0056an asc 0096 asc 0097 asc 010lan asc 0110an asc 0127an asc 0128 asc 0136an asc 0161 asc 0167an asc 0168a asc 0174an asc 1339 asc 1359fd asc Each of the ASCII files has the following format the first line shown here is to identify the field names for these steps and is not part of the file Survey Time Latitude Longitude depth gravity mag residual_mag 3 016 0740 11 82834 131 55832 21 9782666 46040 20 3 016 0750 11 80500 131 56165 24 9782670 46032 28 3 016 0800 11 78334 131 56333 29 9782670 46024 34 INTREPID has powerful batch processing tools which make it simple to process multiple files in a single run The following steps include examples of task specification files which can process all of the data together Naturally before using this feature you would conduct tests using the INTREPID tools intera
12. done either with or without reference to an external dataset eg satellite altimeter data The marine gravity data used in this example has a nominal precision of approximately 2 mGal Location of sample data for Cookbooks Where install_path is the path of your INTREPID installation the project directory for the Cookbooks sample data is install_path sample_data cookbooks For example if INTREPID is installed in C Program Files Intrepid Intrepid4 5 then you can find the sample data at C Program Files Intrepid Intrepid4 5 sample_data cookbooks For information about installing or reinstalling the sample data see Sample data for the INTREPID Cookbooks in Using INTREPID Cookbooks R19 For a description of INTREPID datasets see Introduction to the INTREPID database G20 For more detail see INTREPID database file and data structures R05 Library Help Top 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Library Help Top Moving platform import and levelling worked example 1 Moving platform gravity import and levelling C09 2 lt 4 Back gt Overview 1 Import the raw data into INTREPID line datasets using the Import tool 2 Split the cruises into reasonably straight line segments using the Split Cruise tool 3 Use the Observed gravity to calculate the FreeAir anomaly Convert units from um s2 to milliGals 4 Merge the separate line datasets using the Project Manager s Appen
13. ds side by side If you find errors you may need to examine the raw data more closely Further processing options would be to Examine the raw data for errors Adjust the levelling parameters Remove the line or lines causing the levelling error Use the INTREPID Flight Path Editor to clip sections of lines at ship turning points The acceleration caused by turning may cause gravity errors Library Help Top 2012 Intrepid Geophysics lt 4 Back gt
14. forces linear interpolation everywhere The value has the same units as the Fiducial field WeightMethod In polynomial levelling one part of the process requires fitting a polynomial to the misclosure data A weighting scheme can be specified which determines the relative importance or weight of crossovers in the fitting process 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Moving platform gravity import and levelling C09 11 Library Help Top Library Help Top 4 Back gt Saving and loading crossovers You can save a set of crossovers as a point dataset and load it again during subsequent sessions If you load a set of crossovers INTREPID will use them instead of calculating a new set For a large dataset this can represent a considerable saving of time A crossover dataset also records the corrections which were calculated when it was created If you set all levelling correction methods to No INTREPID will apply the levelling correction recorded in the crossovers dataset To save a crossovers dataset use the statement OutputXOver before the Parameters Begin statement to specify the crossover dataset to be created To load a crossovers dataset use the statement InputXOver before the Parameters Begin statement to specify the crossover dataset to be used Gridding 6 Grid the data and review the grid The task specification job file grid_var job will perform the operation using an alg
15. he INTREPID Split Cruise tool Call the resulting dataset split_geosat DIR This dataset will contain a set of North South lines covering the survey area and a set of East West lines also covering the survey area Importantly this dataset contains crossover control points which are used by the INTREPID MarineLevel tool This split_geosat DIR dataset is provided for you 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Moving platform gravity import and levelling C09 14 Library Help Top Library Help Top 4 Back gt Marine Level 5 Use the Marine Level tool to fit the individual surveys splitnnnn onto the lower resolution satellite based regional gravity using a least squares piecewise polynomial fit Marine Level compares crossovers of the cruise data with the satellite data in the levelling process Up until now we have processed the raw data as separate surveys one dataset per survey Whether we keep the survey data separate or merge all the surveys into a single dataset really depends upon the network adjustment levelling strategy we wish to use In this example we are going to use polynomial levelling to perform a network adjustment of the crossover points for all of the surveys using the geosat dataset as the reference surface Because we are using an external dataset as a reference there is no need for all of our survey data to reside in a single dataset We can process the split nnnn datasets
16. his task contains a Repeat loop you must submit it to the Project Manager To run the task assuming that the task specification file is called append_datasets job use the command fmanager exe batch append_datasets job INTREPID will perform an append task once for each value in the dataset list A new dataset called master DIR will be created from appending all of the splitnnnn datasets together 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Moving platform gravity import and levelling C09 9 Library Help Top Library Help Top lt 4 Back gt 5 Use the Marine Level tool to perform an internal network adjusment using all of the crossovers as control points Marine Level will calculate the crossover points then minimise the misclosure errors throughout the crossover network using the method of loop closure levelling Using the current version of INTREPID you must run the Marine Level tool in batch mode The task specification job levelloop job will perform the network adjustment on the master DIR dataset To run the task assuming that the task specification file is called levelloop job use the command fmanager exe batch levelloop job The Marine Level tool will process the input FreeAir field It stores the levelled data in a new field called FreeAir_loop Process Begin Name marinelevel Comments Intrepid Audit Stamp ReportFile marine_level rpt Pri
17. litnnnn datasets For each Inputs value the Split Cruise tool divides the in the rawnnnn dataset into reasonably straight segments and creates the corresponding splitnnnn dataset The Inputs list is the same as in step 1 above Repeat Begin Inputs Process Begin Name splitcruise Input rawS Inputs Output splitS Inputs Parameters Begin SharpAngleTolerance 50 TrendAngleTolerance 45 TrendDistanceInSamples 10 MinimumSamplesBeforeDrop 10 Parameters End Process End Repeat End The SplitCruise process will produce the following INTREPID datasets spl1it0004 sp1it0053 split0055 split0056an split0096 split0097 split01l10lan split0110an split0127an split0128 split0136an split0161 split0167an split0168a split0174an split1339 split1359fd Convert the gravity field in each splitnnnn dataset from um s2 gravity to mGal free air anomaly values and store the results in a new field called FreeAir The task specification job file freeair job will perform the operation for all splitnnnn datasets in a single batch task Since this task contains a Repeat loop you must submit it to the Project Manager rather than directly to the Spreadsheet Editor tool To run the task assuming that the task specification file is called freeair job use the command fmanager exe batch freeair job 2012 Intrepid Geophysics 4 Back gt INTREPID User Manual Moving platform gravity import and levelling C09 6 Library Help
18. ly Reference dataset OtherDataset split_geosat OtherInputZ split_geosat FreeAir Xovers OutputXover crossovers DIR Parameters Begin Method Begin RunType LevelPolynomial ByCruise No Method End PopulationAnalysis Yes SaveEmptyGroupsInXover Yes DoPseudoFidsAsRecords Yes DuplicateCrossOver_Fid_Tolerance 0 001 MaximumPointSeparation 10000 0 MaximumInterpolationGap 10000 0 Polynomial Begin Width 15 Order 0 MinNpts 2 ConvolveWidth 5 DistanceWeighted No Polynomial End WeightMethod Unity Parameters End Process End Repeat End 2012 Intrepid Geophysics lt 4 Back gt Library Help Top INTREPID User Manual Moving platform gravity import and levelling C09 16 Library Help Top Library Help Top 4 Back gt Parameters RunType Ifthe crossover adjustment is done with reference to an external dataset then the valid options are LevelPolynomial LevelDC LevelLinesDC and LevelDrape If you choose the LevelPolynomial option you must include the following four Polynomial parameters Polynomial Width Use this to specify the number of crossover points to use in the piecewise polynomial used for levelling Polynomial Order Use this to specify the order of the polynomial fit Polynomial Minimum Points Use this to specify the minimum number of crossover points in a line for the levelling adjustment to be applied INTREPID will not attempt to adjust lines with fewer crossove
19. mary input dataset Dataset master DIR InputZ master DIR FreeAir OutputZ master DIR FreeAir_loop Xovers OutputXover crossovers DIR Parameters Begin Method Begin RunType LevelLoop ByCruise No Method End PopulationAnalysis Yes SaveEmptyGroupsInXover Yes DoPseudoFidsAsRecords Yes DuplicateCrossOver_Fid_Tolerance 0 001 MaximumPointSeparation 10000 0 MaximumInterpolationGap 10000 0 WeightMethod Unity Parameters End Process End 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Moving platform gravity import and levelling C09 10 Library Help Top Library Help Top 4 Back gt Parameters RunType Ifthe crossover adjustment is done without reference to an external dataset then the valid options are LevelLoop and LevelPolynomial See Loop closure levelling in Chapter 1 Levelling line datasets in Reference Manual volume 3 for further information about the loop levelling process ByCruise Set this to Yes if you wish to perform levelling by cruise The splitcruise process will generate some lines for which no crossovers are found This option determines whether these lines get levelled or not In levelling by cruise the levelling process is applied to the whole of a cruise and completely ignores the break down into the lines that were created by the split cruise process PopulationAnalysis Set this to Yes if you wish to generate statistical reports for the in
20. orithm suitable for variable density data Examine the grid using a sun angle view to locate levelling errors You may wish to grid the unlevelled data and compare the two grids side by side If you find errors you may need to examine the raw data more closely Further processing options would be to Examine the raw data for errors e Adjust the levelling parameters Remove the line or lines causing the levelling error Use the INTREPID Flight Path Editor to clip sections of lines at ship turning points The acceleration caused by turning may cause gravity errors 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Moving platform gravity import and levelling C09 12 Library Help Top 4 Back gt Moving platform import and levelling worked example 2 Overview 1 Import the raw data into INTREPID line datasets using the Import tool 2 Split cruises into reasonably straight line segments using the Split Cruise tool 3 Convert the gravity field from um s2 gravity to mGal free air anomaly values 4 Prepare a satellite altimeter derived gravity image for use as a levelling reference surface In the following example this preparation has been done for you 5 Use the Marine Level tool to compute and apply a network adjustment using the method of least squares piecewise polynomial fit This method adjusts the data to fit onto the lower resolution satellite altimeter regional gravity dataset This dataset is
21. put dataset s based upon histograms of crossover populations SaveEmptyGroupsInXover Running Split Cruise often results in line segments which have no crossovers Normally crossover datasets do not include line segments with no crossovers Set this to Yes if you wish to include them DoPseudoFidsAsRecords The Marine Level tool requires a fiducial field to calculate the levelling adjustment Usually this would be Time If your data has no fiducial field set this option to Yes in which case the process will create a pseudo fiducial If this is not selected and there is no fiducial the process will use the cumulative distance as a fiducial DuplicateCrossOver_Fid_Tolerance A rejection criteria for crossover points If crossover points occur very close together they can cause levelling problems This parameter sets a minimum tolerance on the allowable distance between crossovers Where two crossovers are within this separation one is rejected The value has the same units as the X and Y fields in this example 0 001 degrees approx 1 km MaximumPointSeparation A rejection criteria for crossover points Where the survey points exceed this separation the crossover is deemed less reliable and is rejected The value has the same units as the X and Y fields in this example 10000 degrees so effectively no limit MaximumInterpolationGap This parameter controls the use of spline versus linear interpolation For example a value of zero
22. r points than you specify here Polynomial Convolve Width After INTREPID calculates the levelling corrections it applies a smoothing convolution filter to the data This filter removes high frequency components of the data Use this parameter to specify the width in data points of the convolution kernel LevelDC Use this option if you wish to make a DC adjustment add or subtract a constant value to the entire dataset such that it best fits the external reference dataset LevelLinesDC Use this option if you wish to make a DC adjustment add or subtract a constant value to individual line segments within the dataset such that it best fits the external reference dataset LevelDrape Use this option if you wish to adjust the input dataset to the reference dataset using piecewise linear interpolation making all misclosures equal to zero 2012 Intrepid Geophysics lt 4 Back gt INTREPID User Manual Moving platform gravity import and levelling C09 17 Library Help Top lt 4 Back gt Gridding 6 Grid the data and review the grid The task specification job file grid_multivar job will perform the operation using an algorithm suitable for variable density data It will read the data from a list of separate datasets avoiding the need to append all of the data together into a single dataset Examine the grid using a sun angle view to locate levelling errors You may wish to grid the unlevelled data and compare the two gri
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