Report on GPS Data Collection in Mexico

Map of Mexico

By Margaret Hart

Meeting
I spent two weeks in Mexico in December, collecting GPS data for a scientific project I and others have been working on for over 18 years. My purpose for this trip was to try to get better control over the locations of previously set surface survey stations and cave entrances in a mountainous region west of Ciudad Victoria, the capital of Tamaulipas. The project manager and I had discussed the goals of the exercise, which included improving the accuracy of the locations of previously measured surface survey stations and cave entrances that were part of the project.

This cave mapping project, Projecto Espeleologico Purificacion (PEP), was initiated in the late 1970s by a group of cavers and speleologists living in Austin, Texas. The PEP caves are located in a major karst area in the Sierra Madre Orientals, the southern extension of the Rocky Mountains that cuts through Mexico. One of the large cave systems in the area, known as Systema Purificacion, is the primary focus of the group. Mapping of the system began in Cueva del Brinco, a cave located in the tiny logging town of Conrado Castillo. I started my Mexican caving there in 1977, on my first trip to Mexico. We explored the upper parts of the cave system - passages named the World Beyond, the Crack of Doom, Mudball Crawl, Locomotive Breath. If this sounds a little bit like something out of Jules Verne, maybe it could be. Another cave in the area, Cueva del Infiernillo, was connected with the Brinco entrance in 1980, making the cave the deepest in the western hemisphere for a few years. The horizontal distance between the two entrances is less than 10 kilometers as the crow flies, but the trip between the entrances takes a minimum of 24 hours to complete, followed by a long and arduous 12 hour hike back to Conrado Castillo. This cave is currently the longest (81.95 kilometers mapped passages) and the 7th deepest cave (995 meters) in the western hemisphere. By comparison, Mammoth Cave in Kentucky is the longest cave in the world, with over 400 kilometers of mapped passages.

Mapping of the cave system has been both extensive and painstaking. Crews use hand held compasses, clinometers, and tapes to make surveys of the cave passages. Surface surveys are completed in the same manner. My job was to check the locations of surface survey stations and major cave entrances and use the most accurate means available to determine their geographic coordinates. This information would then be combined with the data from the cave surveys to give a more accurate estimate of the location, length, and depth of the caves. We could also get a feel for how close some passages were to the land surface.

Setup of the equipment
The equipment list included two Magellan 5000 Pro GPS receivers, a Magellan Trailblazer GPS receiver, a John Chance Omnistar DGPS receiver and processor, two Motorola radios, and a laptop computer for planning and data analysis. I worked with my faithful and patient assistant, my husband Peter Keys, during the data collection phase. Each morning I checked the satellite availability for the best times to collect data. This was my first field experience with the Omnistar equipment, so I spent a few days getting used to it. The Omnistar requires input from either the GPS receiver or a PC regarding the location where the receivers will be operating. The Magellan 5000s did not lend themselves to doing this. I had no easy way to do this task, or so it appeared. The Omnistar manual was somewhat discouraging, but I kept looking at what I had to work with. The Trailblazer did have NMEA download capability, however, and I used it to perform this task.

I then had to get used to the Omnistar and its requirement for a clear view of the satellite it received data from. The area we worked in was covered with heavy stands of mature pines and steep hillsides, and this made satellite acquisition difficult at times. Many of the cave entrances are located on the sides of hills or cliff faces, making satellite reception difficult. Prior planning definitely paid off. I used the laptop every day to check azimuths and elevations. The azimuth and elevation for the Omnistar satellite were fixed at about 150 azimuth and 65 elevation, so I knew this requirement. The only other challenges were the weather and terrain, although at times they were challenges, indeed.

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