Point of Beginning


July 6, 2001
Geographic Calculator, Version 5.0 from Blue Marble Geographics is a multi-faceted utility program for working with coordinates from different systems.

Geographic Calculator aids survey managers in Austin, Texas, with conversions of control monument databases.
Geographic Calculator, Version 5.0 from Blue Marble Geographics, Gardiner, Maine, is a multi-faceted utility program for working with coordinates from different systems. “Working with” is an appropriate phrase, because there are few coordinate chores this package can’t handle. Although the software’s primary function is to convert coordinates from one system to another, there are a number of other useful features, such as forward and inverse of coordinates (including latitude/longitude), batch file handling and entire mapping file conversions. In addition to built-in parameters for thousands of predefined coordinate systems, the software features routines with which users can define their own parameters, creating custom coordinate systems, datums and even measurement units. But rather than using the features of the software as a starting point, let’s begin with some real-world applications, and from there examine the program’s capabilities that make the applications work.

Figure 1. Geographic Calculator set up to convert an AutoCad drawing to a different coordinate system.

Survey Control Point Management

Gary Glover, RPLS, a surveying manager with the City of Austin, Texas, has used Geographic Calculator for several years. Part of Glover’s mission is developing and maintaining a database of control monuments in the Austin area. There are obvious applications for Geographic Calculator software in establishing a common basis for these points, since they originate from various coordinate sources. Examples include converting UTM to State Plane Coordinates, computing State Plane values from geographic coordinates and upgrading values for points originally surveyed in NAD83(86) to NAD83(93), or High Accuracy Reference Network (HARN) values. Geographic Calculator was an easy choice as the appropriate vehicle for all those conversions. But an additional conversion capability came in particularly handy for this effort: the routines for converting an entire AutoCAD drawing from one coordinate system—indeed, from one datum—to another. The previous generation of city monuments was, as might be expected, based on NAD27 datum. An AutoCAD drawing of those monuments, along with a detailed depiction of city streets and landmarks had been maintained over the years. Geographic Calculator was able to read in this old .dwg file and convert the complete drawing to NAD83(93) basis. This kind of conversion, from NAD83 to or from NAD27, should of course only be undertaken with a full understanding of the characteristics of those two systems. For instance, the actual coordinate values for survey control points should probably not be taken directly from the 27 - 83 conversion. For GIS-level mapping, however, the coordinate conversions between the datums is of appropriate accuracy.

To perform the Map File Conversion, as it is called in Geographic Calculator, the user selects an input file from a browser dialog box, along with specifying the “from” and “to” coordinate bases. If desired, the software will identify the output file it creates with the same name as the input file, but with a user-designated prefix. The screen for these settings is illustrated in Figure 1.

Creating Custom Coordinate Systems and Datums

Occidental Petroleum has 18 licenses of Geographic Calculator in its Houston, Texas office. Pat Hagar, who oversees the survey and GIS technicians using the software, says, “We use Geographic Calculator a lot, and for many different purposes. It’s easy to create your own coordinate system. Sometimes, for displaying information on locales that are sensitive or need to remain confidential, we’ll throw in some dummy parameters to create a coordinate system that, if you plotted the points, would end up in the middle of the ocean. Geographic Calculator is very versatile.”

Also in Houston, BP AMOCO uses Geographic Calculator’s user-defined coordinate system capability to create its own “state plane” coordinate systems in areas of sparse control. Using global CORS control for positioning, the company can create a coordinate system from the ground up with its own set of parameters. The software is used extensively for the company’s locative databases of its resources and its exploration.

Petroleum companies find Geographic Calculator to be easy-to-use in creating custom coordinate systems and datums.

GIS Database Maintenance

GIS is, in large part, an exercise in database management. Maintaining an accurate database with point data from many different sources can be a challenge. Geographic Calculator furnishes easy-to-use tools to help keep the data consistent. Batch conversions are possible to and from dBase files, Excel, Lotus 1-2-3 or straight ASCII. Even more important for the GIS user is the Map File Conversion capability discussed above. In addition to AutoCAD DWG/DXF through R2000, Geographic Calculator can make complete conversions of map files in these formats: ESRI Shapefile, MapInfo TAB and MapInfo MIF.

Geographic Calculator is a staple in the GIS world. The University of Georgia’s Center for Remote Sensing and Mapping Science (CRMS) recently conducted an intensive series of educational sessions at its GIS conference. Geographic Calculator was one of the primary software tools used and taught, right along with such GIS stalwarts as ESRI’s ArcInfo.

The Environmental Data Center (EDC) is a geographic information system (GIS) laboratory in the University of Rhode Island’s (URI) Department of Natural Resources Science, College of Resource Development. The Rhode Island Geographic Information Systems (RIGIS) database is stored at the EDC. The RIGIS database is the most comprehensive and detailed of any state in the country and contains information on almost all aspects of Rhode Island’s natural and cultural resources (wetlands, aquifers, soils, forests, land use, topography, historic sites, etc.). Like CMRS, the Rhode Island GIS credits Geographic Calculator as one of its beneficial primary software tools.

Figure 2. Batch files for inverses are easily set up in Geographic Calculator from an Excel spreadsheet.

Project Coordinate Transformation Analysis

Recently, the Texas Department of Transportation was charged with studying the feasibility of combining several contiguous sections of a proposed highway design into a unified system. Each segment was in a different coordinate system: variables included two different State Plane zones, three surface adjustment factors, and both iterations of the NAD83 adjustment (HARN and non-HARN). To make an intelligent decision, it was necessary to manipulate the various groups of coordinates in several ways. One set of operations was simply sorting points into different sequences for different purposes, and performing simple arithmetic upon the values. Those functions suggested a spreadsheet program like Excel. One crucial element was to see how the distances between given points varied as a function of which coordinate system the points belonged to. It was possible to mathematically derive the correct surface distance between given pairs of points, also in an Excel spreadsheet, using the grid scale factor and the elevation. That true surface distance could then be compared to the various inverse distances resulting from different coordinate transformation algorithms. This facilitated a best-fit transformation, producing as a by-product an easily understood listing of the comparisons. The entire operation consisted therefore of bringing data into Excel, manipulating it, opening it in Geographic Calculator, then converting and inversing the coordinates. In the Excel format the points could be sorted into various sequences and operated upon mathematically, while the conversions and batch inverses were a piece of cake for Geographic Calculator. Loading large data sets back and forth between programs could have been a tedious nightmare, but Geographic Calculator’s ability to directly read the Excel files made it easy. Figure 2 shows both the Excel and the Geographic Calculator screens set up to perform a set of batch inverses on a point file.

This is versatile and powerful software. Its file handling smoothness is especially impressive, as is the overall user friendliness. Geographic Calculator’s widespread good reputation seems to me to be well-deserved.