Are compasses accurate enough to perform land surveys?
A popular hiker's compass that features a mirror for viewing the circle while sighting.
For as much as GPS technology is new, the compass is old. Yet the functionality of the compass with its importance to navigation and position remains solid. There will be no contender anytime soon to challenge its role as the most immediately accessible angular measuring device. For angular accuracy, there are far superior devices such as the theodolite. For absolute direction relative to true north or south, only the gyroscope would enter the competition against the compass. Unfortunately, it would be grossly more expensive and far heavier to carry around.
Compasses, unlike GPS, are subject to huge accuracy variations and cannot claim the virtues of being either accurate or precise instruments for land surveying purposes. What they are wonderful at doing is land reconnoitering, at least to an extent sufficient for preliminary orientations.
GPS receivers now have compass-style readouts built into them. They work well enough until the batteries need replacement. A true compass requires no batteries, making it dependable while being lighter to carry around. The basic direction provided by a compass is often more useful than a latitude and longitude, as obtained from a GPS receiver. More than a few times, compasses have prevented gross directional errors and even saved lives.
A compass-style "app" for cell phone, graduated to 10°.
A drawback to compasses is the simple fact they do not point to true north, but are oriented by magnetic fields. Also, their needles, being magnets themselves, will align in the direction of anything ferrous, or iron-bearing. This includes autos and especially the steel in ships and boats. Local iron-heavy rocks, such as commonly found in the mountains comprising the eastern range of San Diego County, can have dramatic effects on compass needles, especially if the iron deposits are nearby and in canyon areas. Tutorials on how to use a couple of the more well-known commercial compasses are to be found here.
The primary correction needed to make a compass point to true north is to rotate its dial--provided the compass is designed with a rotating dial--by the amount needed to cause the needle to be aligned with true north, i.e., the North Pole of the earth. In California, the North Pole is to the left of the magnetic lines of force, which is to say, equivalently, that the north magnetic pole is to the right (and in fact it is centered over northern Canada).
Handheld GPS receivers usually provide both a true north basis and a magnetic north basis. Of course, in order for the receiver to "know" the direction of magnetic north, it must retrieve the declination angle from programmed memory. The chart below, showing the magnetic lines of force across the United States, illustrates the data a receiver uses to calibrate it for magnetic angle readout. The same chart can be used to approximate the local declination for calibrating the compass circle.
Magnetic declination chart for North America, courtesy NOAA. Click map above for full image at NOAA site, 2.35mb png format.
An automated calculation or scaling from the declination map for determining magnetic declination can be useful but of course ignores local effects. A direct observation of the magnetic needle from north, as north is determined by another means such as the sun at high noon or the north star, or even a north-south running road, can be a better way to calibrate for local use.
NOAA's magnetic declination page includes links to historical variations in the declination since the early 1800s. For example, in the San Diego region, the declination was around 12° East (to the right of true north) in 1830, and had increased to 15° by 1930. it now appears to be declining to 12° again by 2030.
A compass face graduated in one degree intervals, reading 280°, or nearly West, in the azimuth angle format (0°-360°).
Compasses were the primary instrument for laying out the land in early US history. The declination, which was sometimes referred to as variation, was sometimes called out in deeds when written in specific reference to magnetic north. In a deed, direction is relative to the basis as stated. Retracing such deeds often meant replicating the variations that existed at the time of the original survey, mimicking the original compass used to conduct directional observations. Compasses were standard technology in George Washington's day, as well as Lincoln's fifty years later. To use them today for the same purposes as these men used them, we would be asking to relive the many land disputes that had arisen over time, due to the inaccuracies of compasses and misunderstandings about their declination angles.
How accurate are GPS measurements?
The blue GPS unit at left, about 4" tall and mounted on a surveyor's "candy cane" rod, is self-contained with its batteries making up most of its weight. The white crown at the top is the built-in antenna. A GPS receiver is basically a power source, an antenna, and a compact bundle of microchips.
GPS satellites transmit bursts of microwaves roughly a quarter meter in wavelength at regular intervals. The waves are picked up by a ground-based receiver that can calculate the trip times from the satellites. These trip times translate into distances, no single one of which is particularly accurate. When received in thousands of repetitions certain errors can be filtered out. Despite the volume of data received, the result is processed in seconds because (1) the signals are very fast (moving at the speed of light), (2) the clocks in the satellites and the ground unit are just as fast, and (3) the computer in the ground unit is also operating at near the speed of light and can easily keep up with all of the data coming in.
There are several errors introduced into GPS positions that limit their ability to pinpoint a location precisely. Some of the more important of these are:
- Atmospheric irregularity
- Satellite geometry
- Clocking errors
- Local signal blockage, reflections
The commonly specified accuracy for basic handheld GPS receivers is roughly 3 meters on the Earth's surface, better than you can effectively plot on a USGS quadrangle sheet. Given that such maps can hardly be penciled to 10 meters when scaling, mapping accuracy becomes a moot point. On the other hand, if you are looking for a geocache somewhere--or a survey monument--an error of just a few meters might leave you searching for a long time.
A professional-grade GPS handheld receiver that has dual functionality: It can display exactly as a hiker's model, but can also collect and log the type of data that can be used later with "post-processing" software to greatly refine its position.
The Earth is about 40 million meters around its equator. Accordingly, in parts per million, a typical hiker's GPS receiver is specified for accuracies of roughly 1 part in 13 million. With respect to the size of the earth, this is significantly better than the best conventional surveying techniques used prior to the advent of GPS–in the 1980s–when it struggled to achieve a tenth of that accuracy. Conventional surveying, as it was two decades ago, would typically cost thousands of dollars to fund the personnel and equipment needed for a state-of-the-art global measurement, and even then with an expectation of maybe 10 meters of accuracy globally. 10 meters may seem like a big error, but with respect to the breadth of the US continent, some 3000 miles, it is equivalent to about a third of a second of arc.
Whereas land surveying-grade receivers need to provide accuracies for the maintenance of the land ownership system, which cannot tolerate errors of several meters, civilian GPS users in the vast majority of other applications can get by with the standard error specifications.
Screen shot from a Triton 2000, a handheld receiver for hiking and geocaching. The satellite screen shows rough orbital locations overhead.
Since the end of SA (Selective Access) during the Clinton Administration, where GPS receivers threw a deliberate error of some 100 meters into positional calculations, there have been improvements in both the number of satellites and where they appear in the sky. Most satellites are moving overhead, but a few are in geostationary orbits: from the surface of the earth, they don't appear to be moving. This helps improve the accuracy of the final calculations made by the ground receivers. These geostationary satellites are called WAAS, for Wide Area Augmentation System. One such satellite is over the Pacific Ocean, with another over the Atlantic. The FAA's WAAS page features 2D real-time views of all orbiting GPS satellites and, through the Google Earth interface, 3D real-time views.
GPS-Why Is Two Better Than One?
The basic measurement principles of accuracy and precision apply dramatically to the use of GPS, especially as observed relative to the size of the earth. One receiver can be used to determine a position to any point on the earth’s surface, but this determination turns out to exhibit a statistical error of several meters. There are many factors contributing to this, atmospheric disturbance and the arrangement of the satellites overhead being two.
When two receivers are employed simultaneously, large scale errors that apply in equal quantity to both receivers can be eliminated in much the same way that an equation stays in balance when the same quantity is subtracted from both sides. A much more precise result for the relative positions of the two receivers can be filtered out of the data received, using special software, when both receivers are collecting data at the same time.