Web Exclusive: Viewpoint

May 29, 2002
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To OPUS or not to OPUS, that's a no-brainer!

Can the local surveyor rely on the accuracy of the results of OPUS (Online Positioning User Service), the system of the National Geodetic Survey (NGS) to provide GPS users easier access to the National Spatial Reference System (NSRS)? Can anyone with only one dual-frequency GPS receiver establish control points for any of their survey projects? Can those control points also be accurate enough to use as bench marks? Can all of this be done without the surveyor doing any post-processing or having any post-processing software? The answer to all of these questions is “Yes.”

How many of us pulled the 200-foot chain out of the truck to check our first top-mount EDM? How many of us pulled out our old reliable total station that had been proven over the years to make sure that our new GPS-RTK system was correct? You guessed it…I had to do some static work just to prove to myself how accurate OPUS data could be.

On several occasions, I had compared one known bench mark or a HARN point with the coordinates generated by OPUS, but I had never compared more than one point. I had been very surprised with the accuracy of the individual experiments, but I was totally amazed when I compared a series of points.

The last experiment I did was so surprising that I felt many surveyors would like to know how easy it is to put every survey they do on NAD83(1993) and NAVD88 coordinates. Even though I used four dual-frequency receivers at one time to do my control points, the results could have been achieved with only one dual-frequency receiver.

I was hired to do some topographic work in an area that covered three square miles. As I looked for control monuments in the area, all I could locate was one second-order bench mark approximately four miles to the southwest of the project area and a HARN monument approximately nine miles to the northwest of the project area. I know, some of you who are GPS static geniuses and post-processing professors will say that this is not the way to do a network for control points. I agree with you, but the time needed to locate three or four more bench marks and control monuments is not within the budget of most topographic projects.

Since I do not like to use a one-point calibration for my RTK work, I wanted to have four control points around the three square miles. I decided to use the bench mark to the southwest as one of my RTK control points and then set three iron pins for control monuments that would encompass the entire project site. A GPS receiver was placed on each of the iron pins. The first receiver was started at 1130 hours. I set this and two other receivers to collect data at 10-second epoch rates. These three receivers were left on the iron pins for approximately three hours. Even though OPUS uses files containing 30-second epoch rates, I wanted to post-process the files between each other and the two control monuments I had found. With the fourth receiver, I occupied the HARN monument for 50 minutes. This monument was over nine miles from the farthest of the three receivers. I then went to the bench mark to the southwest of the project area and occupied that point for 40 minutes. This monument was over six miles from the farthest of the three receivers. Now it was time to retrieve all of the receivers. The last receiver was turned off at 1510 hours. I had more than three hours of data collected on each of the three receivers that was going to be used for the OPUS processing.

For the comparison of the two sets of coordinates, I used Ashtech Solutions (Thales Navigation, Santa Clara, Calif.) for my post-processing. Using the HARN monument as the fixed horizontal position and the bench mark monument as the fixed vertical position, I calculated the following coordinates for the three iron pins I had set:

Pt.# Northing Easting Elevation
1 415579.882 1474611.015 1486.039
2 431203.768 1474929.341 1466.082
3 431296.914 1469068.908 1462.684

After creating RINEX files from the three receivers that stayed stationary for at least three hours, the three files were E-mailed to OPUS. Although sometimes it takes 24 hours for OPUS to process the files, I have sent the files after the 24-hour period and received the results within five minutes. I was E-mailed the following coordinates from OPUS for the three iron pins I had set:

Pt.# Northing Easting Elevation

1 415579.779 1474610.968 1486.204
2 431203.741 1474929.298 1466.385
3 431296.880 1469068.817 1462.956

I know some will say that this is not accurate enough control for some survey projects, and I would agree. However, the relative difference seems to be minimal for the three square miles. How would you like to have every project that you have ever done be relative to each other within 0.08 feet horizontally and 0.14 feet vertically?

Check out the NGS OPUS system at http://www.ngs.noaa.gov/OPUS/index.html.

Note: The original version of this article was published on the Oklahoma Society of Land Surveyors (OSLS) website at www.osls.org.

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