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  The American Surveyor     

Software Review: Geographic Calculator, 6.0 Print E-mail
Written by Joseph H. Bell, LS   
Sunday, 29 February 2004

A 2.954Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

The Geographic Calculator, from Blue Marble Geographics, is the kind of utility program that allows the average surveyor to swim around in the geodetic pool without drowning. They might have called it the geodetic surveying calculator. I have been using the Geographic Calculator for many years and found it not only extremely easy to use, but very reliable. Every time I review a program that purports to do geodetic transformations, I check it out with The Geographic Calculator.

The Geographic Calculator used to be handy for those few who were required to step into things like the State Plane Coordinates. Now, with the increasing use of municipal GIS and the increasing use of dual frequency GPS RTK systems, the Geographic Calculator needs to be in the hands of the great majority of surveyors. Important tools have been added to the calculator with each new addition. The most recent of these is a new Graphical User Interface (GUI) for Windows XP users. There is a new polynomial best-fit method for transforming local or non-standard coordinates to any standard coordinate system (such as NAD83) stored in the geocalc.dat database. This includes a "display registration errors" graph that displays your best-fit accuracy.

In addition, the Geographic Calculator can handle vertical transformations from any ellipsoidal height to orthometric (sea level) heights using GEOID99, GEOID96 and AusGeoid98, which are available as free downloads from the website (www.bluemarblegeo.com).

They have added Eckert IV and Eckert VI world projections and the Michigan GeoRef coordinate system. There are some very technical tools dealing with the International Terrestrial Reference Frame (ITRF). Because the earth wobbles, True North is never exactly the same. In order to have an orderly coordinate system, True North has to be constant so it is fixed at a particular date. Measurements must be corrected back to that date. If you have been using a Continuously Operating Reference Station (CORS), then you may have had to deal with this problem.

Figure 1 shows the basic window. There is a Menu Bar and a Power Bar under which are three tabs: Interactive Conversions (shown), Point Database Conversions and Map Conversion. You can define any two systems and convert forward or backward. You can set the units, the input formats for latitude and longitude and the vertical reference. You get the NGS data in latitudes, longitudes, and ellipsoidal heights in meters. You need to have the data in State Plane Coordinates and orthometric (sea level) heights and in US Feet. You do not even have to type in anything but the coordinates. The rest of the stuff you just select from a drop down list. You can always convert in either direction.

If both points are in the same system, you can do an inverse if you are in a State Plane Coordinate system or if you are in latitudes and longitudes. You get geodetic distance, geodetic azimuths forward and back, grid distance and grid azimuth forward and back.

You can also do what geodesists call a Forward calculation (what we surveyors call a traverse). Put in the coordinates of the Station and click on Forward and it will ask you for a distance and an azimuth. You check whether it is Grid or Geodetic. You can actually run a traverse because you can do a Forward in either direction.

Figure 2 shows the Tab: Point Database Conversion. This allows batch conversions of entire databases and is customizable to just about any format. Better than that, you convert a whole directory of files with a single key stroke. You can save each format you use in a file so that you only have to define each format once. Name the file and recall it when you want to convert another database in that format.

There is a very interesting function in the Geographic Calculator that allows you to make custom conversions. The function is correctly called Rubber Sheeting. Most every surveyor I know has a lot of work in Local Coordinates (such as 10000, 10000). In order to take advantage of GIS technology, or to integrate your work into a single standard projection, or simply just to satisfy the local authority who is trying to build and maintain a GIS, this function is marvelous. You need to have a minimum of three points with local coordinates and standard coordinates such as State Plane coordinates in an Excel file (you can use as many as you have). The Geographic Calculator creates a Best Fit set of conversion parameters which you can then use to convert all the local coordinates on that job into State Plane Coordinates.

Figure 3 shows the table read into the Geographic Calculator. There is a B order station (AIR), some first order stations, and a manhole (MH) (navigation coordinates from a handheld GPS). I created the local coordinates for the control station from the coordinates on the MH.

Figure 4 shows the geodetic coordinates and the errors in Northing and Easting as well as latitudes and longitudes. You can see how the navigation coordinates warp the conversion. When I left the M H out of the conversion factor, the fit was slightly improved, but remember that the local coordinates were based off the MH navigation coordinates.

Figure 5 shows the file conversion. The first BM is the navigation position and the conversion shifted it about 10 feet north and 10 feet east, closer to the true State Plane position. Now that you have created the conversion parameters for this job, you can use it in all three tabs, Interactive Conversions, Point Database Conversions and Map conversions. Each conversion file you create is added to the list of standard conversions so when you come back to it, you just pick it off of the list.

Figure 6 shows the Tab: Map File Conversion. This allows you to convert an AutoCAD drawing in one coordinate system to an AutoCAD drawing in another coordinate system. I love to use my navigation handheld GPS receiver for preliminary work, but the coordinates come out in latitudes and longitudes in decimal degrees. I can output a DXF file but it is a bother to try to import other survey data or images. It is very simple, however, to convert the AutoCAD DXF or DWG to State Plane Coordinates. You can also convert to and from in Blue Marble Layer files (.bml), and ESRI Shapefiles (.shp). This gives you tremendous power. For instance, my City and County GIS is by ESRI and many shape files are available.

The ability to convert AutoCAD drawings from your local coordinate system to WG S84 latitudes and longitudes and then to any standard coordinate system using your custom transformation parameters makes it possible to bring all of your previous work into a single standard projection. There is now available a simple GIS for surveyors that allows you to create a real GIS of all of your work. That makes all of your work instantly available for research, for re-issue to client or agency, or just to review how your business is progressing.

The File Menu drop down list shows: Open Coordinate Systems Settings, Save Coordinate Systems Settings, Record Interactive Conversions, Print, Open Point Database Settings, Save Point Database Settings, Exit and License manager. You download the Geographic Calculator for free and if you like it, you can call and get a key and use the License Manager to create a fully licensed copy from the downloaded version.

The Options Menu drop down list shows: Unit Display settings (which brings up a sub-menu allowing you to configure the units and display type of all of the various types of information), Linear Unit Definitions, Angular Unit Definitions, Ellipsoid Definitions, Datum Definitions, Coordinate System Definitions, Text File Schemes and Preferences (General, File Folders, Point Database [for custom conversions] and selecting the various editions of AutoCAD for conversion).

The Windows Menu drop down list shows: File Viewer and Best Fit Plot. The File Viewer is a generic viewer that allows you to view AutoCAD drawings and E S RI Shapefiles. The Best fit plot plots the errors in the custom conversion file. You can select to have it plot automatically each time you make a custom conversion.

The Help Menu drop down list shows: Contents (F1), License Manager Help, Obtaining Technical Support, Blue Marble Geographics on the Web, Download the Latest Version, Online support and About the Geographic Calculator.

The Power Bar Icons (from left to right) show: Print, Define Coordinate System, Define Datum, Define Linear Units, Define Ellipsoids, Define Angular Units, Define Text Schemes, Display Vector File Viewer, Display Error Plot and Show Help Contents.

The only limitation I could discover is that AutoCAD map conversion does not yet work with AutoCAD 2004. A nice addition would be to convert between the different map formats instead of conversions limited to within a single format (i.e., AutoCAD to Shapefiles). Needless to say, this is one of my favorite workhorse programs. It won't cost a cent to download it and see for yourself.

Joe Bell is licensed in California and New Mexico. He has been reviewing software for surveyors since 1982.

A 2.954Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE


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