API, AOPL working to standardize GPS system

Nov. 9, 2009
A joint American Petroleum Institute-Association of Oil Pipe Lines team is working to develop standard global positioning system coordinate submittal guidelines in an effort to reduce potential errors in locating oil and gas pipelines.

A joint American Petroleum Institute-Association of Oil Pipe Lines team is working to develop standard global positioning system coordinate submittal guidelines in an effort to reduce potential errors in locating oil and gas pipelines.

One-call centers throughout the US increasingly include GPS coordinates as part of a one-call notification by an excavator. GPS coordinates can help accurately locate proposed excavations. Reliance on GPS readings without a clear understanding by all parties concerning important settings and nomenclature options, however, can lead to a false sense of security and actually introduce new errors into the one-call notice process.

An excavator should avoid simply reading numbers off the screen of whatever equipment he or she picked up at the local sporting goods store to the one-call center operator. One-call centers, similarly, should avoid accepting at face value whatever coordinates are given over the phone and simply entering them into the computer system, and line locators should avoid assuming the GPS information they receive is in a certain format or uses a certain datum.

This article addresses three potential sources of GPS error: coordinate nomenclature, datum selection, and accuracy, in the hope its use will further understanding of these issues by all parties involved in the one-call process.

A number of factors affect the accuracy of GPS readings, including the number of satellites, the sophistication of the GPS unit, etc. The accuracy of particular information (i.e., ±29 ft, 58 ft, etc.), is obtained from typical GPS units and frequently is shown on the main screen along with latitude and longitude coordinates. Users widely hold accuracy information to be the only source of error when using GPS coordinates, resulting in a false sense of security. This article shows accuracy to be only one source of error, with other less understood sources perhaps having a much larger effect on absolute accuracy of a depicted location.

Coordinate nomenclature

One common nomenclature uses degrees, minutes, and seconds, dividing the world into 360 degrees with each degree composed of 60 min, each minute composed of 60 sec, and each second recorded in decimal format. Degrees, minutes, and seconds are denoted by °, ', and ", respectively, such as: –30° 53' 27.95".

Another, equally accurate nomenclature uses degrees and decimal minutes, without seconds (30 seconds being equal to 0.5 decimal minutes). A third popular nomenclature simply uses decimal degrees (30 min being equal to 0.5 decimal degrees, and 30 sec being equal to 0.008333 decimal degrees).

A GPS coordinate submitted in degrees, minutes, seconds is much different than one submitted in degrees, decimal minutes, which is much different than one submitted in decimal degrees. The numbers may look the same, however, and many times a set of coordinate numbers could be feasible in several different nomenclatures.

This GPS screen shot shows results obtained by plotting a particular coordinate with degrees and decimal minutes (Fig. 1).

Fig. 1 plots the GPS coordinate N 29 50.30 W 95 50.50 (with degrees, decimal minutes).

Fig. 2 plots a similar-looking coordinate (N 29.5030 W 95.5050, with decimal degrees).

The same values used in Fig. 1, but placed in a GPS system with decimal degrees, produce a much different result (Fig. 2).

These plots use the same series of digits (295030 and 955050) and both lie within the greater Houston area, regardless of nomenclature used, but are more than 30 miles apart. The fact that many GPS units fail to show on their main screen which nomenclature is being used can confuse one format with another or create difficulties in determining which a particular GPS unit is using, leading to potentially huge mapping errors.

Datum selection

A separate but related issue is the selection of datum in the GPS unit. In this context, a datum is a theoretical model of the earth's surface the GPS coordinates reference against. A typical Garmin GPS unit can be set to more than 100 different datums, 11 starting with NAD 27. Each of these datums is different from the rest, reflecting different models of the earth's surface, ovality, etc.

Few people are familiar with what datum selection means or how to select the correct one. They simply use the default datum to which their unit was set when purchased (or however the previous user left it). Datum selection on a GPS unit usually requires the user to drill down through several menus of settings, and the correct datum to select is far from obvious. The same GPS coordinates, however, will actually lie on different points on the earth when using different datums.

The National Imagery and Mapping Agency has published a document (NIMA TR8350.2) listing parameters for many geodetic datums. Some have exotic sounding names like Astro Tern Island (FRIG) 1961 or Pico de las Nieves, while others carry more mundane names like Geodetic Datum 1949 or Arc 1950. Setting the GPS receiver to one of these datums shifts the measured coordinates anywhere from a few meters to as much as 1,000 m from the nominal WGS 84 datum of the GPS system

Plots of the center of the Texas State Capitol dome, for example, with the WGS 84 datum at longitude 97:44:25.19 West, and latitude 30:16:28.82 North as the reference, often vary by more than 500 m from the intended location even with the same coordinates.

The difference between common datums in North America is usually rather small (on the order of dozens of feet, not miles), but even this degree of error could be important for one-call purposes. Both NAD 27 and NAD 83 datums are widely used in North America. The NAD 83 datum is newer, but many companies and government agencies (such as the US Department of Interior, and the US Geological Survey) still use NAD 27, and many maps use that datum.

Even with the same nomenclature, differences in the underlying GPS datum can still lead to differences in location. The NAD 83 datum produces the location result shown here for the given coordinates (Fig. 3).

If someone submitted GPS data with the coordinates shown in Fig. 3 and plotted it with the NAD 83 datum (virtually identical to the WGS 84 datum, the default for most GPS units), the results would be as shown.

With the same coordinates and nomenclature as Fig. 3, but the NAD 27 datum, a different location appears (Fig. 4).

But the NAD 27 datum (the default for USGS topographic maps) with the same data represents a point on the other side of the street in a different back yard (Fig. 4). This difference, though small in absolute terms, could lead a utility operator to mistakenly clear a one-call which in the belief the proposed excavation would not affect its facility.

Path forward

All parties involved in the one-call process should understand the importance of GPS accuracy, nomenclature, and datum selection, and establish procedures to avoid confusion concerning these items. A recent API-AOPL Damage Prevention Workshop recommends considering development of standard GPS coordinate submittal guidelines to increase the confidence in and reliability of GPS coordinates submitted by excavators to one-call centers, and transmitted from one-call centers to locators.

Training all one-call centers on this issue with a standardized coordinate submittal criteria, and using a help guide to talk people through how to verify or change the datum and coordinate system settings on common GPS equipment, would also eliminate much of this confusion. An industry group intends to work with the Common Ground Alliance to modify existing CGA best practices and establish new best practices as necessary to address these issues for all involved parties.

The Virginia Pilot Project for using GPS coordinates in damage prevention activities used NAD 83 as the datum and decimal degrees as the format. The National Pipeline Mapping System also prefers NAD 83 and decimal degrees.

To maintain consistency with these previous initiatives, the API-AOPL work group specifically recommends one-call systems adopt NAD 83 and decimal degrees (with a minimum of six decimal place digits) as the standard GPS nomenclature for one-call use. The WGS 84 datum (the default setting for most GPS units) is virtually identical to the NAD 83 datum throughout North America and should also be accepted. The work group also recommends one-call centers obtain and share the stated accuracy of the GPS readings when taking information from excavators.

The author

W.R. Byrd ([email protected]) is president of RCP Inc., an engineering and regulatory consulting firm, and is a consultant to the API-AOPL Pipeline Performance Excellence Team working to resolve this issue. He holds a BS and MS in mechanical engineering from the Georgia Institute of Technology (1981, 1982) and is a registered professional engineer in Texas, Louisiana, Mississippi, and Alabama.

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