From the Field
Members of the Oklahoma Society of Land Surveyors (OSLS) wanted to provide a technical session to supply the quarterly continuing education program and many mentioned the photos of the Initial Point of Oklahoma in a recent issue of our newsletter. The OSLS decided to put the IP on the National Geodetic Survey (NGS) map through some GPS sessions.
Planning the SessionAfter deciding upon a date for the event, the planning began. Test triangulation for the sessions were plotted and discussions began with the NGS about the best use of time to get the most accurate position possible, given the limited one-day excursion to the Initial Point.
We decided that an Online Positioning User Service (OPUS) position would provide the most accurate position on the point since we were having trouble getting units committed to also run sessions on High Accuracy Reference Networks (HARNs) in the area. We decided to run a four-hour plus OPUS session.
The OPUS processor is provided by the NGS and all that is required is the occupation of a point with a dual-frequency receiver for a minimum of two hours and then the submission of the file to the NGS via their website. It is a very eloquent solution for users needing an accurate tie to the National Spatial Reference System (NSRS). Usually within 10 minutes or so you have an E-mail in your inbox with the ITFR and NAD 83 position along with some statistics on the reliability of the position.
Another NGS point was very close to the IP so we ran some single-frequency units on this point and its references at the same time. The point, “Indian,” was set in 1954 and listed the IP as nearby. Strange that the IP was within 50' but they made no physical tie to this point on the PID sheet. Indian was listed as third order and its references would make a nice addition to the other reference markers for the Initial Point.
The next step in the planning process was acquiring the original notes for the setting of the IP so we could verify its location, although many have used it with little doubt. The IP was first used by Sledge Tatum, U.S. surveyor, in June of 1897 when he began the survey of Oklahoma. He listed the instrument used as a light mountain transit with a solar attachment. The called-for stone was listed as a sandstone 54" x 18" x 18" set firmly in the ground. Subsequent notes reported the stone in 1925 laying on its side and reset in the correct location. The size and weight of this stone would have made moving it far very hard.
JD and Shawn from East Texas and Jimbo from Louisiana, frequent posters on the POB message board (www.rpls.com), arrived late on Thursday evening in Lawton to visit the Initial Point. Without going into the Thursday night details, you can imagine that a good time was had with many wonderful survey tales told until late into the night.
Pilgrimage to the PointFriday morning arrived much too quickly and we drank our coffee in the driveway at 4:30 a.m. while waiting for our ride. As we drove to the site of the IP with the sun gradually climbing into our eyes, much talk centered on the lack of canopy in our area. It was a reminder that GPS is an excellent tool, but many areas are just not GPS friendly. JD and Jimbo took to calling our little bit of southwest Oklahoma the “RTK Heaven.”
We met up with the rest of the surveyors and completed the pilgrimage to the Initial Point. Several crews began scrounging for evidence of references to the Initial Point while others began the recovery of the NGS tri-station.
Upon deciding the IP was in the same physical spot, we set up our first receiver on this point. Our units were several years old and used daily by our crews. The first unit would not log data. This was no problem since we had brought a spare, but the second experienced the same fatal errors as the first. Later we learned that both units were purchased together and had the internal lithium batteries fail at the same time. Murphy strikes again.
Another user, Tony Felder, PLS, had a unit we could use, so we fired it up and began logging data. Not until later, when trying to download did we find out that this unit had been full of data and only a couple of minutes was stored. Scratch our goal of getting an OPUS session!
Shawn Billings set up his Promark (Ashtech/Thales, Santa Clara, Calif.) on the NGS tri-station Indian, while I set up one of our Trimble (Sunnyvale, Calif.) 4600s on reference point 2 to Indian. We wanted to stay with fixed height rods so we had Shawn set up his Promark using our Trimble 2.0 m rods. Some debate occurred as to whether this was now a “green” session since none of us had attempted mixing blue and yellow before.
Both of these single frequencies performed perfectly and completed their sessions. With no data in the way of dual frequencies to use and only four hours each in our single-frequency units, we had to resort to Optimistic Real Geodetic Intersections (ORGI) processing. ORGI processing incorporates the use of multiple Continually Operating Reference Stations (CORS), as many as can be found to surround the unknown point and as equally distant from it as possible, and constrains the adjustment to fixed positions on all CORS sites.
While not a technique approved by any manufacturer or the NGS, we have had remarkable success in repeating measurements using the CORS provided data and processing baselines anywhere from 50 km to 150 km with repeatability in the centimeter range. This would be our first big daylight test and we only had four hours of data to work with instead of our usual eight.
As the units logged data, we began a contest to see who could draw the best certified corner record (CCR) to the Initial Point with the main prize being a large bottle of Oklahoma Stumpwater Ale from JD. Michael Porter, PLS, took top honors and after the CCR is drawn it will be distributed for all attendees to file this historic corner.
As the Oklahoma summer sun crept upwards around noon the typical near 100-degree weather began to take its toll on all the attendees. We thought we had enough data so we called the sessions to an end.
The rest of the day and the next were spent on a site-seeing tour of our lakes and mountains. Sadly, the weekend came to a close and I bid the intrepid bunch farewell. We had become friends on the rpls.com board and wound up cementing this friendship in person. These mini gatherings are a wonderful way to find out about other surveyors and their unique areas. JD and Shawn, being from Texas, had never seen a GLO stone while I learned a great deal about using GPS in East Texas and Los Angeles.
For a more detailed explanation of OPUS, see the POB web exclusive article “To OPUS or not to OPUS, that’s a no-brainer!” .