In  Search  of  ARL5

June 17, 2001:  It's a hot Sunday afternoon and thousands of people are crowding into Six Flags Over Texas for a full day of entertainment.  Meanwhile, a few blocks to the east, I've taken my family to a lesser-known attraction:  We're hunting for a pinpoint location known as ARL5, a reference point which has been determined with millimeter accuracy by the National Geodetic Survey.

The NGS has determined that the ARL5 reference point is at exactly
32° 45' 32.49931" North, 97° 03' 36.99084" West, at an altitude of 562.1 feet

ARL5 is just one of many reference points known as Continuously Operating Reference Stations (CORS).  As you can see, these stations are scattered all over the country.  Most are located in population centers, but some (like WSMN and MDO1) are located at very-high-tech research centers in the middle of nowhere.

Regional CORS map - Click to enlarge

The one we're searching for on this occasion is near the intersection of Interstate 30 and State Highway 360, directly south of DFW Airport and about halfway between Dallas and Fort Worth.  The Highway Department has an office here, which I believe was once the central office for the DFW Turnpike, a toll road which paid for itself and then became I-30.  Out on the lawn, southwest of this building, there is an inconspicuous wooden stake in the ground.  In fact it's so inconspicuous you can hardly see it in this picture.  Near this stake is the top of a concrete and steel assembly which is flush with the ground.  Finding this thing was made possible by a detailed description of its location which is part of the data sheet found on the CORS web page.  The data sheet indicates that the marker is 53.5 feet south of the green sign on the left side of this picture.

Looking northeast: an inconspicuous wooden stake in the 
foreground draws little attention

What great fun!  At last I have located ARL5, which at this point appears to be just a metal plate in the ground, surrounded by concrete.  (Ha!  Look at all those tourists driving by on their way to Six Flags.  They don't know what they're missing!)

I've found it at last! (You're looking east.)

Upon closer inspection this metal plate appears to be an "Access Cover":  Just a little metal door under which you might expect to find a water meter or valve.  However the perimeter of the cover has an inscription describing it as a "Geodetic Control Mark" and warning about the penalties for disturbing it.  Let's look inside.

Looking down: The access cover

After carefully opening the cover, I was astonished by the simplicity of what I found.  This is just a piece of pipe sticking up from the bottom of a small shallow hole in the ground, a well about five inches in diameter, with a small pointed stainless steel rod protruding from the center.  Evidently the tip of the pointed rod is the exact reference point.  According to the data sheet, this stainless steel rod goes at least ten feet into the ground.  There are a few scraps of orange plastic material wrapped around the sleeve, like the kind of material used for surveyors' flags, most likely left behind after a recent measurement.

Looking inside: Opening the cover reveals the reference marker

Unfortunately I didn't have a ruler with me to place next to the object, to establish its scale more accurately, but this picture gives an idea of its approximate size.  It was at about this moment that I realized that I might have set off an alarm someplace just by opening the cover.  But that's just a healthy sense of cautiousness, of course, not paranoia.

I wasn't disturbing it, honest!

You might wonder why this reference point exists, and why the federal government cares so much about its exact location.  From what I've been able to find out on the internet, it is reasonable to infer that continuous GPS measurements are made somewhere near this marker, and the GPS estimate of the marker's position is published so that other GPS users in the area can get an idea of the GPS position error in real time.  In other words, if GPS says this reference mark is two feet east of its actual position, then all GPS measurements made at that moment should be adjusted two feet to the west.  I suspect that this service is for the benefit of survey crews working on highway construction and other public works projects, and that any benefit to the public is just coincidental.  Of course I have reached this conclusion without having interviewed any official about it, and without any formal education in this subject, but I wouldn't be surprised to find out that I am exactly right.

Update:  The people at NGS Headquarters got wind of this web page, (how did that happen?) and Dr. Dennis G. Milbert contacted me via email, offering this explanation:

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.

(As it turns out, the point that I found was ARL5 A, which is located at exactly
32° 45' 30.04048" North,  97° 03' 43.02741" West,  at an altitude of 533.51 feet.)

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.

Best wishes.

Dr. Dennis G. Milbert
Chief Geodesist
NOAA, National Geodetic Survey, N/NGS
1315 East-West Hwy., SSMC3, Room 8635
Silver Spring, MD

Obviously this new information calls for an overhaul of this page.  I need to rush out to Arlington again and take some pictures of ARL5 B as well as the actual GPS reference antenna, which as it turns out, is the real ARL5.  Watch this page for additional updates and new photos, coming soon.

Update:  More photos are now available.  See Return to ARL5.

I intended to mention benchmarks, AC6326, GPS, ARL5, the Global Positioning System, and the Continuously Operating Reference Station.

Custom counter developed in-house

Document location
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