RTK Connection: Will GNSS networks replace GRS monuments?
August 1, 2009
Permanent geodetic reference system (GRS) monuments have long been the standard for locating positions on, below or above the Earth’s surface.
However, the reported location of a fixed GRS monument--the latitude, longitude and ellipsoid height, which can be represented by a state plane coordinate and an orthometric elevation--is only completely accurate at the time the data are collected. The relevance of the monument decreases over time. To maintain the usefulness of the monument, it must be reobserved and remeasured. For this reason, some people believe that roving GNSS networks offer a distinct advantage.
Making AdjustmentsIn my home state of Missouri, the Department of Natural Resources has assisted counties in establishing GRS monumentation throughout the state. These monuments are bars and aluminum caps with carsonite posts to assist in recovery. The DNR, with assistance from local surveyors, collected GPS data on those monuments, processed and adjusted the data, and then reported out the final adjusted locations for those monuments. At the time, they also established azimuth marks for these monuments to give geodetic or grid bearings. These monuments have been used by surveyors to establish state plane coordinates for subdivision corners, determine geodetic references for projects, recover section corners and lines, and any other use that needs a geodetic reference.
Over the years, the DNR has reoccupied monuments, adjusted the results and reported out new locations. The new locations came from better measurement techniques, more data entered in the adjustment formula, and movement due to tectonic activity. Such adjustments have resulted in some confusion in the surveying community. Two surveyors will come up with different results on the same subdivision corner because one survey referenced the original data sheet for the GRS monument and the second survey referenced the new one. (This almost always results in one of the surveyors wanting to return his or her brand-new GPS system because it clearly was giving bad results.) However, if the first survey was completed before the new adjustment on that monument and the second survey was completed after the new adjustment, then both surveyors are correct.
A true GNSS network consists of permanently mounted receivers that constantly collect raw satellite data and then send the data to a central server. This central server receives the data, monitors the integrity of the data sets, then analyzes and processes the data in real time. This analyzing and processing in real time is the key to a good network. The network can monitor the base stations and adjust for any errors introduced in those base stations--such as movement or tropospheric effects. The central servers finally push out RTK corrections to rovers, which can then calculate their position almost instantly. That position is immediately relevant and can be used as a geodetic reference, and that geodetic reference can be used just like traditional GRS monumentation.
Accuracy and InterpretationIt’s important to understand that I’m not talking about legal-entity monuments such as section corners, property corners, right-of-way markers, etc. With these monuments, the monument itself is more important than the location of that monument. While legal-entity monuments might have a recorded location, that location is simply an attribute of that monument, not its definition. A section corner is defined by the stone as it was originally placed and referenced. Even if its placement and reference was a little sloppy and not quite where it was supposed to go, it is still the corner. If a county surveyor recovers that stone and replaces it with a bar, cap and monument box, the bar becomes the legal entity and thus the corner. If the county surveyor then records its location, the bar is still the definition of that corner, not the location.
Locations are not absolute; they are subject to many different factors. Ten different land surveyors might come up with 10 different locations for the same bar based on their measurement techniques, skill levels, definitions of “location,” and the ever-popular random error. But all 10 of those surveyors will agree that the bar is the section corner (well, maybe 9 out of 10). With this agreement that a physical object recovered and supported by acceptable evidence is the corner, then land ownership has a solid foundation. Will GNSS networks replace these legal-entity monuments? No. But I do think it will replace the rest of them--the monuments that are simply references used to determine locations.
The roving geodetic reference has two big advantages over static monumentation. The first is mobility. Being able to set a reference on or near your job is very convenient. The monuments in Missouri are set about three to five miles apart, so you might have to traverse a few miles to bring a geodetic position to your job. With a network rover, you will have a geodetic position right on your jobsite almost instantly. The second advantage is the real-time adjustments that networks process. This results in your position being as accurate and relevant as possible. Traditional monuments are only as good as the last adjustment, which could be years old.
There is one disadvantage to networks: The positions reported by them are not “blessed” by a government agency. Traditional monumentation has a seal of approval from a state department of natural resources, the National Geodetic Survey or another government agency. These agencies are the ones that report the position, and they stand behind that position. A good network will stand behind the correction that it sends out, but it cannot fully guarantee the resulting position because it has no control over the rover being used or the operator of the equipment. As a result, the surveyors have to take responsibility for the position. They will need to use their experience and surrounding evidence to determine if the position is correct. (This kind of sounds like real, traditional surveying, doesn’t it?)
Since reference monuments are just that--references--I believe that GNSS networks will replace them in the near future. These networks can report accurate, timely positions more efficiently than any other method. They allow surveyors to set local geodetic references to use just like a traditional monument, but they are more up-to-date and relevant.