| GPS Consumer Series: Using GPS with Offset Information: Laser Rangefinders, Part 2
By Chuck Gilbert Introduction
A GPS-based data collection system can be a very handy tool. In itself, a GPS receiver will not solve all your data collection needs. However, coupled with the other tools in your "tool kit," such as laser rangefinders, GPS can save you a lot of time and money.
This combination of technologies is very useful for recording the location of objects that are difficult to access or dangerous to occupy. In situations where it is difficult (or impossible) to place the antenna directly on the object to be mapped, a laser rangefinder can provide a distance and direction from the antenna to the object of interest. A well designed GPS system will be able to accept this offset electronically and to apply it to the GPS position so that the user can later plot or export the true location of the object that could not be occupied. (See the sidebar for a brief recap of offsets and how they apply to GPS data collection systems.) In February, this column was an introduction to the use of laser rangefinders in conjunction with GPS. This month this column addresses some aspects of GPS and laser rangefinders that are more subtle. Depending upon your application, some of these special considerations will be very important, and others may be irrelevant. Are you 2D or 3D?
Many people think of an offset as simply a distance and a direction on the compass. This is sufficient if you are concerned only with horizontal coordinates. If you also require elevation, then a little more information will be necessary. In order to compute the difference in elevation between your GPS and the object of interest; you will need to know the inclination at which the distance measurement was made. Many hand-held laser rangefinders also supply inclination automatically. If your application requires elevation data, ensure that the system you use provides inclination information/measurements. How is the offset expressed?
For the sake of usability, it may be important to consider how offset information is expressed to the user.
There is more than one way to describe a three dimensional offset. For example, in addition to the compass bearing, the offset could be described in terms of a horizontal distance and a vertical distance. Alternatively, in addition to a compass bearing, the offset could be described as a slope distance and an inclination. A well designed GPS system will allow users to enter and review the offset values in either form. Otherwise, a user in the field is forced to do impromptu trigonometry to convert a slope distance into horizontal and vertical components (or vise versa). The potential for error while doing trigonometric functions in the sun/snow/wind/traffic is very high.
Additionally, it is occasionally handy to be able to toggle from one form to another. There are times when a horizontal distance is desired (such as a span length between poles). However it is not always possible to shoot a horizontal range with a laser. (Perhaps, for example, because the lower portion of the pole is being blocked by a building). In this scenario the user can easily shoot the top of the pole then simply view the offset in terms of horizontal and vertical distance. Tricks and Traps
The most common error source when combining laser rangefinders and GPS is compass error. The range measurement of a laser is generally quite dependable, even when shooting at a non-reflective target. The accuracy of GPS after differential correction is also quite robust. However, there are a variety of reasons that a compass may give an incorrect bearing. The most commonly encountered problems are summarized below. Magnetic Corruption
Users should be aware that the compass in a laser rangefinder adheres to the same magnetic principles as a hand-held magnetic compass. Therefore, if the compass in the rangefinder is held too near an object that contains ferrous metals, the compass will be magnetically influenced and will provide an incorrect reading. One of the most common rogue influences on a compass in the field is the automobile of the user. However, other magnetically influential materials can be much more subtle, such as the steel reinforcement hidden within concrete (especially bridges). Be aware also of the potential of magnetically influential materials in the GPS receiver itself. It may be necessary to keep a certain distance between your GPS receiver/antenna and the compass on your laser. The potential problem of magnetic corruption is usually quite easy to avoid in the field as long as the user is aware of the issue and is paying attention. Magnetic Declination
Another potential problem is magnetic declination. The Earth's magnetic pole is not in the same location as the Earth's rotational axis. (The rotational axis is defined as true north.) Therefore, from most locations on the planet, the direction to magnetic north (e.g. the direction that a magnetic compass points) is different from the direction of true north. This difference is known as the magnetic declination. True and magnetic north can vary from one another by as much as tens of degrees.
The problem is simply a matter of making certain that both the laser rangefinder and the GPS are configured properly. If the laser outputs bearings with respect to magnetic north and the GPS is expecting input with respect to true north (or vice versa) you have a problem. It does not matter which north reference is used, as long as both pieces of equipment that are exchanging data are in agreement about using the same reference. This problem is also easily avoided as long as the user is aware of the issue. The Gunfighter
Compasses take time to settle down. After a magnetic compass is rotated to a particular orientation, it takes a moment or two for the magnetic element of the compass to stabilize on the correct reading. You will always get better results if you take your time with a measurement as opposed to whipping the compass out of your pocket and reading the bearing before the needle has stabilized. The same principle is true for laser rangefinders that contain an internal compass. If you whirl around and shoot the pole while diving for cover it is likely that the accuracy of your bearings will be poor. Experienced users will often take several shots at each object simply to confirm that they get about the same answer on a consistent basis. This is usually not an inconvenience since it usually takes only a second or so for a laser range measurement. Maximum range
There is a direct relationship between the accuracy of your offset position and the accuracy of your compass. The longer the offset distance, the more any compass errors are accentuated. Many of the rangefinders on the market today have a compass that is specified as accurate to only a fraction of a degree (typically about one half to one quarter degree). However, the maximum range of these rangefinders can be as great as many kilometers when shooting at a reflective prism. It is important to realize that if your compass has an error of 0.5 degrees and you shoot the range to an object 300 meters away; you will introduce about 2.6 meters of error to the final position. If your GPS system is rated at sub-meter accuracy you should consider the maximum distance that you shoot as well as the specification of your compass.
The table below provides a guideline for error contributions due to compass uncertainty and offset distance (rounded up to the nearest five centimeters). The table above illustrates the degree of positional uncertainty that will arise based upon the uncertainty of the compass and the distance of the range that is being measured. Summary
This month's column addresses a few more of the issues to consider when evaluating the combination of GPS and laser rangefinders. The major points are summarized below:
• If your application requires elevation data, make certain that the system you purchase can accommodate that need.
• Take note of how the offset data is displayed. Make certain that you can view the offset data in the most convenient form for your application.
• There are several issues related to compasses that the user should be aware of before recording offset data. Practice on some known targets first and pay attention when working with a compass.
Next month, we will consider several additional capabilities that may be found in a laser rangefinder compatible GPS systems; including aspects of the post-processing software that can make or break the usability of such a system. About the Author:
Chuck Gilbert has over a decade of experience as a GPS user. He has been employed as an applications engineer for Trimble Navigation since 1989. Back | 
