Surveying in the Marsh

September 6, 2000
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Web exclusive: Using GPS in Hixtown Swamp in northern Florida.

Second growth of the Cypress swamp area.


As technology has developed over the ages, the surveyor’s tools and instruments have become more complex, more efficient, more precise and more expensive. As these new tools have been perfected and their usefulness demonstrated and proven, costs have come down, and become accepted and used by the current crop of surveyors as the new “industry standard.” These tools vary from the knotted ropes and rods of the early practitioners, to compasses and chains, to transits and steel tapes (still called chains by many purists), to thedolites and EDMs, to total stations and data collectors, up to the “tool of the day”—Global Positioning System, or GPS.

As each new tool is added to the toolbox, it never quite replaces all of the previous ones. They are used to make surveyors’ jobs easier, better, and hopefully more profitable. At the Ocala, Fla., office of Berryman & Henigar, we recently threw open our toolbox and used many of these tools—including GPS—for a survey in the Hixtown Swamp Conservation and Recreation Lands (CARL) project for the Florida Department of Environmental Protection.

Coming across a log bridge and water moccasin.

Not a Normal Worksite

The Hixtown Swamp CARL project, located in Madison County, is one of the largest cypress-dominated basin swamps in northern Florida. The total project encompasses roughly 23,000 acres, including thousands of acres of cypress swamp and freshwater marsh and over 21 archaeological sites, which may include a Spanish mission site.

There were four ownerships listed as essential parcels in our project area. Our task was to survey portions of the 10 General Land Office (GLO) sections that contained these four parcels to determine the locations of the corners controlling their boundaries in order to better quantify the area, land usage and improvements. We were to produce a boundary survey of our project area, which included location of substantial improvements within the parcels and any evidence of occupation pertinent to the parcel boundaries.

The majority of the parcel boundaries were section lines that showed little evidence of having been revisited by surveyors since the original surveys of the townships in the 1820s by Deputy Surveyor James Drake. The south boundary was along two miles of the Tallahassee Base Line.

Conventional ground traversing meant that miles of line would need to be cut with machetes, bush axes and chain saws to reach the vicinity of the original corners. Less than 30 percent of the project area was upland and the project area was used primarily for agriculture and a private hunting area. During the early phases of the project we were asked to stay offsite until 10:00 a.m. to allow for morning turkey hunting. A farming lease included multiple ownerships, and fencing was not a good indicator of possession lines.

The wetland areas contained some open marsh, but more than half was thick cypress swamp. Conducting the survey in drought conditions was a mixed blessing. We were still able to utilize our airboats in the open areas, but some of the normally submerged corner evidence was now out of the water. Unfortunately, the low water also brought a higher concentration of unfriendly reptiles into the remaining ponds. The remaining swamp was a thicket of ti-ti, vines and briars, and in some places the understory vegetation was so thick the field crews could actually climb up and walk across the top of it, 10 feet above ground level.

As luck would have it, most of the bad stuff was along the section lines that needed retracement. Progress slowed to 600 feet per 10-hour day in some places, with field crews walking or boating into the lines with their equipment and spending the entire day in dense swamp. The interior project area was one of the most difficult sites we have ever encountered in Florida and it became apparent that project completion would require a more innovative approach. This brings us back to the use of GPS.

Side tieing a section corner. (Tripod legs were set on top of fence posts that Berryman & Henigar surveyors set.)

Getting Down and Dirty with GPS

The first use of GPS was to establish the primary project control network. We used the existing control network, which had recently been completed as a joint effort between the Department of Environmental Protection, Suwannee River Water Management District, Department of Revenue and local counties (approximately on a 6-mile grid). With our in-house GPS (Leica SR 9500, Leica Geosystems, Norcross, Ga.) in the rapid static mode, we established GPS pairs along the project perimeter at approximately 3-mile intervals. With this control in place, we were able to run ground traverses to tie in the right of way lines of US 90 and the CSX Railroad along the northerly end of the project area; the right of way lines of Interstate 10 (which runs about a half mile south of a portion of our southern boundary), and the adjacent section corners along the remaining boundary lines. With this information, data from right of way maps and information provided by some local surveyors, we were able to calculate search positions for the section corners to complete the project. We computed the intersection of our existing control traverse with the calculated section and proceeded to open those section lines to allow access to and search for the needed section corners.

Using our hand-held GPS units (Lowrence Model 212, Lowrence, Tulsa, Okla.) and the calculated search coordinates, we proceeded to search for section corners, whose positions had been perpetuated by surveys following the original GLO survey of 1825. The hand-held units consistently put us within 5 to 10 feet of the computed location of the section corners. We then opened clearings in these areas to establish additional control to refine our search efforts. The corner positions were either re-established by recovering the GLO references or set based on single or double proportion.

We then moved into the next area of GPS usage on this project. We returned to our Leica 9500 units and added the radio links to use the equipment in the Real-Time Kinetic (RTK) mode. There are two aspects of the RTK that made it the proper tool for this portion of the project. First was the ability to know, without any post-processing, whether the point was GPS-able and to know when the point’s position had been resolved to an acceptable positional tolerance. The second aspect of the RTK was as a tool to “stake out” the point location for monumentation if no other evidence was recovered. These two RTK abilities allowed us to attempt to prove the obliterated corners or set the remaining lost corners. This was accomplished with relative ease in the open airboat-accessible areas. By opening clearings 10’ in depth and using our tall 25’ prism poles we were able to perform the same tasks in the more dense areas. As an independent check of the points set in the RTK stake out mode, we ran a separate rapid static session on those points using different project control. These two values were compared back at the office to minimize the possibility of bad stake out data and/or human error. These two independent results matched within a centimeter, giving us confidence in our results.

The Results

Once all of the section line locations were resolved, we were able to proceed with the final mapping, eventually leading to the timely submission and approval of the product. A total of 42 section and quarter-section corner positions were established with State Plane Coordinates. Our previous wetland mapping is currently being updated to refine the parcel areas based on the new survey information.

The survey tools for this project ranged from a staff compass and steel chains to search for corner accessories, to total stations and data collectors, to Leica 9500 units in RTK mode to accurately determine where the corners fell in Hixtown Swamp.

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