GPS Q&A: Industry experts answer reader's GPS questions

Q. What's a datum? - A.S. Ann Arbor, Mich.

A. John C. Bohlke, Sokkia Corp.: A datum is a reference from which positions can be calculated. The Global Positioning System references a mathematical model of the Earth, the World Geodetic System Datum of 1984 (WGS84). The State Plane Coordinate System references the North American Datum of 1983 (NAD83) or the North American Datum of 1927 (NAD27). The North American datums provide accurate models for positional calculations when measuring in North America. In order to best represent the local area, many other localized datums exist throughout the world. Therefore when computing coordinates based on GPS, the data must be converted to the appropriate datum.

Charles Branch, Ashtech: For the purpose of GPS surveying, a datum is a set of constants specifying the coordinate system used for calculating coordinates of points on the Earth. There are eight constants that define a datum: three to specify the location of the origin of the coordinate system, three to specify the orientation of the coordinate system, and two to specify the dimensions of the reference ellipsoid.
      A comparison to a locally defined coordinate system commonly used in surveying may help clarify. Commonly, a local coordinate system may be defined for a small project by randomly defining a point close by as the origin. This could be a boundary point of a land parcel. Coordinates are assigned to this point such as 0, 0 or 500, 500. In contrast, the origin of a datum is often, but not always, defined as the center of the Earth (termed a geocentric datum), where the coordinates are 0, 0, 0. Next, in the local system, a direction is defined in order to orient the two axes of the coordinate system. In most cases, with a datum, orientation of the three axes is roughly defined as follows: the Z axis begins at the origin (center of the Earth) and runs parallel with the rotation axis of the Earth piercing the surface in the north pole region; the X axis is perpendicular with the Z axis piercing the Earth at the point on the equator where the meridian line (line along surface connecting north and south pole) through Greenwich intersects the equator; and the Y axis is perpendicular to both the Z and Y axes, placing it roughly in the same plane as the equator. Finally, the local system assumes the area within which the survey is being performed is flat. This induces very little error when working in small areas. Many datums are global datums which must take into effect the true shape of the Earth to maintain accuracy. Therefore, a mathematical surface is created which best fits the shape of the Earth. This is the reference ellipsoid.

Herbert P. Colomb, Corvallis Microtechnology Inc.: In general a datum is a reference surface or point from which other measurements are made. Within the mapping community we describe the horizontal datum in terms of an ellipsoid with defined semi-major axis and flattening with a point of origin and a reference azimuth from that point. In the past datums were used to describe portions of the Earth, such as countries or continents. Today with the use of satellite derived information we are able to describe the Earth in a single datum, World Geodetic System, 1984 (WGS84).

Andrew Hurley, Leica Inc.: A datum by definition is any numerical or geometrical set of quantities which serve as a reference of base for other quantities (Geodesy for the Layman, NOAA Reprint, July 1985). In Geodesy we are dealing with two types of datums; horizontal and vertical. And now with the introduction of satellite positioning systems we have satellite datums.
      It is important to remember when referring to a horizontal datum that it is not just an ellipsoid. A horizontal datum will use an ellipsoid as reference and include the two parameters that define it along with a number of other parameters. These parameters include a latitude and longitude of an origin point, the geoid separation at the origin, and a line defining azimuth. The most commonly used datums used in the U.S. are NAD27 and NAD83.
      A vertical datum is any surface that heights can be referenced to. A vertical datum on a national scale is normally established by recording readings from tidal gauges located around a coastline and averaged over an 18.5 year period. This yields Mean Sea Level which for most vertical datums becomes reference for all heighting. The most commonly used vertical datums in the U.S. are NGVD29 and NAVD88.
      With the common use of satellites for positioning we entered a new era of datums. Our datums now become three-dimensional and are referred to as satellite datums. An ellipsoid is used as reference so as latitudes and longitudes can be obtained. The origin is located at the center of mass of the Earth. The datum is defined by a number of permanent tracking stations, a selected geopotential surface, and some constants such as Earth rotation and speed of light. The most common satellite datum is WGS84.

Q. Using GPS, how can I map a place/object that is inaccessible or dangerous to occupy? - E.G. Cupertino, Calif.

A. Bohlke: Resource-grade GPS receivers typically incorporate an offset function that allows users to map objects that are difficult to occupy. A GPS antenna can record information at an accessible location while the user enters the offset to the object of interest. Offsets can be identified by entering a combination of azimuths and distances from one or multiple accessible point(s). Depending upon the desired accuracy, the user can estimate offsets or measure them using conventional tools such as a tape, chain, compass or handheld EDM. The software computes the actual location of the object using the GPS information in combination with the user-supplied offset information.

Branch: The easiest way is to use a laser range finder (LRF) and a compass. You first lase the feature of interest and determine its range from where you are standing. You then determine the azimuth to the feature using the compass. You then combine these two measurements with your GPS position and compute the position of the feature.
      The more this can be automated, the easier this task is. You can buy LRFs with built-in electronic compasses. These go quite a way toward automating the process. You can be totally automatic if you have software that combines the range and azimuth to the feature with your GPS position.
      Some caveats: 1. You cannot use a compass when you are lasing features from inside a vehicle or near any strong electromagnetic source (powerlines, etc.). 2. Don't forget ergonomics: carrying both LRF and GPS equipment can be cumbersome.

Colomb: A quality GPS/GIS data collector should have the ability to collect an offset point. The observer records a position that can be safely occupied and then estimates the distance and azimuth or uses a hand-held laser rangefinder to obtain the slant range and azimuth to the object of interest which is automatically entered in the data collector and will be displayed as an offset point along with the reference position by the post processing software. This procedure can be used for a staic point or in kinematic mode to collect or polygon.

Hurley: The easiest way to map an object or feature that is inaccessible by GPS is to utilize routines present onboard a GPS system. Simple routines that are present include entering point offset information or executing COGO routines. You can specify the direction and distance from the GPS antenna to the object, or you can compute the object based on other measured locations stored in the GPS receiver. A simple sketch and supplemental measurements may be required.
      One of the things I see today is that we seem to forget that other survey equipment exists to undertake such tasks. What is important to remember is how easy it is to incorporate external measuring devices into our GPS. Too many people are thinking of GPS as the one and only answer to all our survey needs. We should be thinking of it as a tool that should be used alongside the measuring systems. Sometimes the high-tech solution is not the most practical or efficient.
      When the situation arises, consider first the accuracy with which this remote object has to be located and then examine the measuring tools available. Whatever tool I choose, I need not rely purely on GPS. Rather, one must be able to integrate GPS measurements with others to make the survey easier and more economical.

About the participants:
John C. Bohlke serves as GPS technical product manager for Sokkia Corp. in Overland Park, Kan. He may be reached at 913-492-4900 or 800-4-SOKKIA in the U.S. (phone) or 913-492-0188 (fax).
Charles Branch is a GIS marketing manager at Ashtech in Sunnyvale, Calif. He may be reached at 408-524-1400 (phone), 408-524-1500 (fax), or e-mail: [email protected]
Herbert P. Colomb, Jr. is a general manager at Corvallis Microtechnology Inc. in Corvallis, Ore. He may be reached at 541-752-5456 (phone), 541-752-4117 (fax), or e-mail: capt [email protected]
Andrew Hurley is a GPS product specialist at Leica Inc. in Englewood, Colo. He may be reached at 303-799-9453 (phone) or 303-799-4809 (fax).

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