Dear Mr. Dart,|
Thank you for the "head's up" on your web page, "In Search Of ARL5".
It looks like you had a very good time, and your intuition is correct regarding the uses of geodetic control.
We did want to mention that what you found was not exactly the National CORS site ARL5. What you found was one of two Texas DOT reference monuments, ARL5 A or ARL5 B.
The actual ARL5 is the bottom (antenna reference point) of a Trimble antenna,
model # TRM22020.00+GP, on top of a tower.|
We did some checking with Texas DOT, and were able to get a photo of the tower/antenna placed on our web site:
click on "Photo" on the left hand menu.
Naturally, I'm sure you have questions regarding how one might use an antenna vs. a ground monument.
The ground monument is a traditional method of locating a mark. Surveyors would start at a known control point, take survey measurements (for example, angles from a transit or theodolite and distances from calibrated tapes or Electronic Distance Measurement, EDM). They would then ultimately connect to a different control point, and check their results against the geodetic control. If everything matched, the surveyor could feel comfortable in his or her work. For more exacting work, the survey data are taken with numerous redundancies, and the data set is then put through a least-squares adjustment, where the geodetic control points are used to establish the reference coordinates.
Your intuition is correct in that the ground monuments help a surveyor bring a data set into conformance with our national system of coordinates (known as the National Spatial Reference System -- NSRS).
The system is much the same with GPS. And, here, we are talking about the geodetic quality GPS, the centimeter accuracy stuff. The geodetic GPS receivers are operated in pairs (or sets), and the errors due to orbit, clocks, ionosphere and troposphere are cancelled through a differencing process.
So, you see, the GPS mode (initially) was much the same. Set up a GPS antenna on a known point, take the second one to a new point, and survey. Instead of theodolites and EDM, surveyors used GPS.
Enough of this was going on, it was soon realized that it was better for everybody, if permanent, continuously operating GPS receivers could be established nationwide. This would support single-receiver operations within the footprint of a CORS. And that is the antenna on the tower (the receiver is inside the building).
We establish the ground monuments just in case something happens to the CORS antenna. A new one can be set up, but it will never be exactly in the same place. We can find the new coordinates very quickly through short survey ties from the ground monuments. In addition, sometimes the towers sink or tilt, and we can verify that motion through repeated short survey ties from the ground monuments.
Dr. Dennis G. Milbert
NOAA, National Geodetic Survey, N/NGS
1315 East-West Hwy., SSMC3, Room 8635
Silver Spring, MD
Document location http://www.ae5d.com/arl5.html|
Created June 30, 2001,
at the end of GPS Week 1120.
Updated slightly on October 28, 2004.
Broken link repaired on April 23, 2009.
Page design by Andrew K. Dart ©2009