by Kurt Luebke, PLS, CFedS
Perhaps you've stumbled across a benchmark while looking for a geocache. Maybe you happened upon the Benchmark Hunting forum
on Geocaching.com and thought it sounded like a fun side hobby to geocaching. Whatever has triggered your interest, odds are that you'll soon discover (perhaps the hard way) that the difference between SCALED and ADJUSTED coordinates on a mark's National Geodetic Survey (NGS) datasheet can literally be a country mile--or at least, a good part of one.
In case you aren't already aware of this, the benchmark database on Geocaching.com is a snapshot taken in 2000 from the database of survey control monuments compiled by the NGS. The logs on Geocaching.com offer valuable information about which marks have been found by geocachers; they often also include photos of the found marks. However, if you really want to get into benchmarking, you should become familiar with the up-to-date NGS database, which you'll find on the NGS website
The NGS database, which provides information on more than one million geodetic survey monuments, is used by surveying and engineering companies, government agencies, and state transportation departments to help control the infrastructure across the United States. Survey monuments allow these agencies to ensure that roads, sewers, waterlines, bridges, etc. that go from one community to another connect at the same elevation or location. For instance, when there are two entities drilling, blasting, and boring from opposite ends of a long tunnel project, without proper survey monuments or systems, they could end up with two tunnel segments that don't meet up properly.
Jefferson Pier marks the location where Thomas Jefferson originally planned to construct a monument to George Washington. Today, it serves as both a horizontal and vertical control station. You can read more about it in Lasting Impressions.
There are two types of control data in the NGS database: horizontal (latitude and longitude) and vertical (elevation). The location of both horizontal and vertical monuments is determined to varying degrees of accuracy, called orders.
The most common designations you will see for horizontal or vertical accuracy are SCALED and ADJUSTED, with adjusted readings being the more accurate. For elevation, you may also see the VERTCON option, which is more accurate than a scaled elevation, but not as accurate as an adjusted elevation.
• A horizontal monument is surveyed to have an accurate (ADJUSTED) position.
• A vertical mark has a precise (ADJUSTED) elevation.
* When a monument is used for both horizontal and vertical control, both sets of measurements will be ADJUSTED.
Horizontal control stations
Looking at the following excerpt from the NGS datasheet for survey station PX0413
, we can determine several things.
PX0413* NAD 83(1993)- 44 58 05.17938(N) 109 27 54.16990(W) ADJUSTED
PX0413* NAVD 88 - 3363.16 ( /-2cm) 11034.0 (feet) VERTCON
The elevation (NAVD 88) is VERTCON, which means that the elevation is not precise. The horizontal location (NAD 83) is ADJUSTED, so it is highly accurate.
STOCKAID, a USGS survey mark in Wyoming, is an example of a horizontal control station.
These days, surveyors can achieve adjusted accuracies by sending surveyed data from a survey point to the NGS for computation by their Online Positioning User Service
. OPUS calculates the station's position in relation to at least three high-accuracy Continuously Operating Reference Stations
. As their name implies, CORS sites gather data 24 hours a day, providing post-processed survey data available throughout the United States and North America.
Using a consumer-grade GPS receiver, you could expect to get within 10 feet of this survey monument, as long as the satellites are not blocked by trees, vegetation, or buildings that can degrade the GPS readings. Any time that you see ADJUSTED horizontal values, you should be able to walk right up to the marker (within the limitations of the accuracy of your GPS receiver).
By the way, if you look for a mark based on an NGS datasheet, remember that professional surveyors use the degrees/minutes/seconds (DDDMMSS) format for coordinates. This is different from geocaching, which uses degrees/minutes/decimal minutes (DDDMM.MMM). There are three easy ways to use your consumer-grade GPS receiver with NGS datasheets:
• If the survey mark you're seeking is in the Geocaching.com database, its coordinates will be expressed in DDDMM.MMM on that site.
* You can change the settings on your GPS receiver to DDDMMSS, then enter the NGS datasheet coordinates directly into the receiver.
* You can use an online site to convert from DDDMMSS to DDMM.MMM. See the Resources
section of Caching Now for links.
Vertical control stations
Now, let's look at a monument that's being used for vertical control.
PY1130* NAD 83(1986)- 44 22 10. (N) 110 34 53. (W) SCALED
PY1130* NAVD 88 - 2426.979 (meters) 7962.51 (feet) ADJUSTED
The datasheet for PY1130
states that the location (NAD 83) is SCALED and the elevation (NAVD 88) is ADJUSTED. Generally, survey marks that were created as vertical (elevation) marks have only scaled coordinates (horizontal values). When the agency that established the survey mark did not know exactly where they were located, they looked at a U.S. Geological Survey (USGS) topographic map and tried, as best they could, to determine where on the map they were. Once they marked that position, they used a scale and determined the latitude and longitude for the location.
CVO 86-15 is a vertical control station placed by the Cascades Volcano Observatory in Yellowstone National Park.
Officially, the NGS states that scaled positions (in the United States) are within six seconds--or approximately 600 feet--of the actual location. However, these scaled survey marks may be up to one-half mile from where your GPS indicates. So when you're looking for a mark that has only a scaled location, you'll want to get to the area with your GPS, then use the location description on the datasheet to actually find your benchmark.
One thing to note, elevation is the weakest component of a GPS measurement. That is especially true with consumer-grade GPS receivers. As a test, you can compare the elevation on your GPS receiver to a vertically adjusted monument. Your reading may differ by as much as 100 feet from the datasheet elevation--or you may find it to be within one or two feet. The elevation reading on your GPS receiver will vary while you are standing there, and may be even more different if you return at a later date. Luckily, even when you're looking for a vertical control station, the most important pieces of information for finding it are the scaled horizontal coordinates and the written description, so the inaccuracy in your receiver's elevation reading should not be a problem.
There are many reasons to join in the fun of benchmarking, including pondering the historical significance of old marks, and enjoying the beauty of the monument locations: mountains, parks, cities, oceansides, deep forests, and bridges. Many survey stations are actually structures rather than disks. These include the Washington Monument, capitol domes, and historic water tanks and water towers. Your benchmark searches create a map of your travels. Additionally, you are helping to map our nation's infrastructure by locating marks that may not have been found for many years. Whatever your reason for searching out survey monuments, enjoy them and treat them respectfully. Thank you for helping professional surveyors and agencies protect this valuable resource for our country.
Kurt Luebke's surveying career has taken him to 16 states across the western US, including multiple trips to Alaska. He is licensed in Montana, Oregon, and Idaho. In 2007, he was in the first class of 69 land surveyors from across the U.S. to complete the Bureau of Land Management's "Certified Federal Surveyor" training and testing program. He is currently the vice president for the Montana Association of Registered Land Surveyors, and has been a member of the Surveyors Historical Society for the past six years.
Even after working all day or all week, Kurt enjoys getting out and searching for old, unusual, historical, political, and inaccessible survey monuments across the U.S. He feels it is his way of paying tribute to those who preceded him.
Originally published on January 10, 2008