GeoTalks Express – Session 6 Questions & Answers

The sixth of Blue Marble’s GeoTalks Express online webinar series entitled What’s New in Geographic Calculator 2020, was conducted on May 27th, 2020. During the live session, numerous questions were submitted to the presenters. The following is a list of these questions and the answers provided by Blue Marble’s technical support team.


Does Geographic Calculator acknowledge the ~2m difference in WGS84 to NAD83 origins yet?

Yes, Geographic Calculator has non-zero-shift coordinate transformations available between NAD83 and WGS 84 for many areas of use.


Can we take a lidar dataset in a projection with elevations in NAVD88 using Geoid12A, can we strip off that Geoid and add in Geoid18 utilizing this tool?

Yes, a 2-step vertical transformation can be performed (NAVD88 to WGS 84 ellipsoidal height via Geoid 12A and then back to NAVD88 via Geoid 18).


Can you briefly explain the proper way to use time-dependent / Date conversation inputs?

Enter the source and target epoch for most time-dependent coordinate transformations. Enter the target epoch for both the source and target dates for 14-parameter transformations. This will perform a transformation between the target epoch and the reference epoch. Note that starting with Calculator 2020, the dates are not prefilled to encourage users to be thoughtful about date selection as time-dependent transformations become the norm.


I take it that the Point Database Best Fit employs methods of least squares?

Yes, the control points can be fit with affine, or 2nd-5th order polynomial functions.


Can you convert a LAS file in a US State Plane 83 grid coordinate system to a local ground-based system?

Yes, a fitted coordinate system can be derived from control points recorded in both coordinate systems with the Point Database Best Fit job.


Would this work with 3D formats too? Such as 3D shapefile or 3D DWG/DXF also with 3D LiDAR data?

For context, we were discussing vector file conversions when this question came in.  The answer is yes, all of the jobs in the Geographic Calculator use both 2D and 3D coordinate systems.  If you have a 3D shapefile or any type of file that supports heights, you can perform conversions on those.


Is it possible to explain Special Features DEM? If any DEM is in a different coordinate system, how can we convert that DEM into other coordinate system in Geographic Calculator?

DEM files convert just like any other file.  You set the horizontal and vertical coordinate systems and select the appropriate Geoid to get between those surfaces.  DEM files may either be written back out to a DEM format or to flat colorized raster images.


When can we perform coordinate transformations – between different data sets when they are opened by the  Global Mapper or on the final data processing step?

In Global Mapper, transformations are done on the fly when data is loaded, but this only affects the data inside the GM session and not the source files that are brought in.  To permanently transform the data you must also export the layers to new files.  The projection set for the workspace in Configuration>Projection will be used for those outputs.


Are the SWARM data used in the IGRF13 and WMM2020 models?

Yes, the European Space Agency’s SWARM satellites are used in both the IGRF13 and WMM2020 geomagnetic models.


Can GeoCalc be used to help establish an unknown coordinate system if the site location is known?

The Point Database Best Fit job allows you to derive a polynomial best fit coordinate system between a known and an unknown (local) coordinate system if you have control points recorded in both systems.

The Projection Recovery Tool (part of the Georeferencer) allows you to estimate a known coordinate system that fits data recorded in an unknown system from a set of reference points.


GM will do that quickly too – is there any difference in accuracy please     Geographic Calculator often has more coordinate transformation options than Global Mapper, depending on the source and target datums, and area of use? ie EN to lat long.

Geographic Calculator has more coordinate transformation options than Global Mapper, depending on the source and target datums, and area of use.

In the case of a projection change, Geographic Calculator generally uses the same formulas as Global Mapper; however, Geographic Calculator has additional specialized projections available.


Can Geographic Calculator import custom a geoid file?

Geographic Calculator supports 16 established grid-based transformation formats.  If your file were in another format that is one of the many formats supported by Global Mapper, a “Blue Marble Gridded Data” (BGD) file can be created there and then exported to Geographic Calculator. There are also eight additional vertical transformation methods including unit change, height/depth reversal, offset and slope, and US VDATUM.


Is the Geographic Calculator covered by the standard Global Mapper license?

Geographic Calculator and Global Mapper each require a separate license/subscription. This includes accessing GeoCalc Mode in Global Mapper.


Can you input files of many coordinate pairs for inverse calculation that outputs a file with the results or does it work only for single point pairs?

Yes, the Point Database Forward Inverse job allows you to perform Forward or Inverse calculations on tabular/spreadsheet data. Excel, ODS, text, and several other formats are supported.


Can I convert an ASCII table with single line vectors defined by coordinate pairs to a shapefile?

The Point Database jobs have the option to write output to a number of vector formats, including shapefile, where one pair of columns is selected for the vector geometry (and all columns in the table are saved as attributes). Therefore, this vector output would contain point features.

If you have multiple columns (coordinates) per row that represent the vertices of line features, the Multiple Conversion feature in the Point Database Conversion job allows for the conversion of these coordinate columns in a single process. Writing this out to an ASCII file, this text output can easily be loaded into Global Mapper as one line feature per row.


Seven parameter 3D transformations?

The Geographic Calculator Datasource contains many 7-parameter coordinate frame rotation and position vector rotation transformations (as well as 3-, 4-, 6-, 10-, and 14-parameter transformations, among others).


NAD83 vis NADCO has accuracy 0f 0.15.  What is the accuracy of the NADCON5 transform?

NADCON5 accuracy varies by region and also within the various combinations of shifts used.  Overall accuracy has been improved, but NADCON5 is now able to associate individual accuracies for a point, rather than publishing one loose statement for the entire coverage area.  We are still working on how to bring this reporting capability into Geographic Calculator.  Also note that NADCON5 and NADCON results are not intended to match.


How can I use this Geographic Calculator for Nigerian Survey?

The Geographic Calculator Datasource contains the following Nigeria-specific geodetic datum, coordinate systems, projections, and transformations:
Datum: Minna
Geodetic coordinate reference system: Minna
4 Transverse Mercator projections: Nigeria East/Mid/West-Belt and WGS 84 / TM 6 NE (offshore)
1 Vertical datum and coordinate reference system: Lagos 1955
15 Coordinate transformations from Minna to WGS 84 (11 Geocentric, 3 7-parameter position vector rotation, 1 multiple regression equation)


How can you help us to include NIGERIAN IMAGERY in Global Mapper?

If you have your own image data, Global Mapper supports a wide variety of formats. There is also a built-in link to the “World Imagery” dataset. Global Mapper supports the Nigeria-specific Minna geodetic coordinate system and the Minna to WGS 84 (2) [EPSG 1168] geocentric transformation. In GeoCalc Mode, access to all the coordinate systems and transformations in the answer to your previous question would also be available to you.

Leave a Reply

Your email address will not be published. Required fields are marked *