| GPS Consumer Series: Using GPS with Offset Information
By Chuck Gilbert Introduction
GPS is not the panacea for GIS data collection and mapping problems. GPS is simply one more tool for your tool kit. There are times when GPS is by far the most efficient means of collecting the type of data that you need, and there are times when other data collection methods are more efficient.
Historically, the situations where GPS has been least effective have been locations where the view of the sky is obstructed, such as under very dense trees or in locations where most of the sky is blocked by either buildings or terrain. The reason that GPS has been less effective in these environments is that a GPS satellite signal is easily blocked by such obstructions. Additionally, a GPS receiver is typically designed to compute the location of it's antenna. This means that in order to capture the location of an object, the user is usually required to go to that location. It is not always practical to physically visit the location of every object that you wish to map. For example, you may wish to map the gauging station in the middle of a stream, or a manhole in the middle of the street. Offsets
The ability to apply offsets to features is an enhancement that has recently appeared in GPS-based data collection systems. This is the ability to record both a GPS position as well as an associated distance and direction to the object of interest. This ability is useful for recording the location of things that cannot be conveniently visited or occupied. For example, suppose you wish to record the attributes of a power pole that is on the other side of a steep, narrow ravine. Rather than risk life and limb climbing across the ravine, users can use the offset capability to stand (and record the position) at one side of the ravine and simply record the offset (distance and direction) to the pole on the other side. (More in a moment about where/how the distance and direction are obtained.) The same principle applies to recording features in dangerous situations such as on and near busy roadways.
Additionally, offset capability can be used to help reduce the travel time associated with field data collection. In many applications, the user spends more time traveling from location to location than actually recording data. The offset function can allow a user to stay at one location and record the offsets and attributes of hundreds of features; then pack up and move to get dozens more from the next site.
For the entry of offset information distance and direction can either be estimated by the user, or obtained more accurately from a hand-held ranging device. There are several commercially available laser rangefinders that provide not only distance, but also azimuth and inclination. What supplies the offset information?
There are several issues that ought to be considered when evaluating a GPS unit data collection system that is capable of utilizing offsets. The most fundamental aspect of such a system regards what (or who) supplied the offset data.
In an ideal system, two components, a GPS and a rangefinder (usually a laser), are totally integrated. That is, the laser is capable of RS-232 output and the GPS is able to accept such an input from the laser. This scenario requires that the laser is capable of also providing compass bearing and perhaps inclination. With such a system you can simply pull the trigger on the laser and the offset distance and direction are automatically transferred to the GPS unit and applied to the GPS position. In a totally integrated system, when the user views (or otherwise outputs) the location of a feature, the correct, shifted position is displayed (computed by combining the original GPS position with the offset). As a result, the maps and output of such an integrated system accurately represent the true locations of the features of interest.
An alternative way of recording an offset is not as foolproof as the automated system described above. In this scenario, the GPS unit is not capable of accepting offsets digitally. Therefore the offset details are manually entered via the keyboard. The user obtains an offset through any means available (perhaps even from a measuring tape and compass) then manually types the distance and bearing into the GPS unit. For the sake of accuracy, a well designed system will allow the user to obtain an offset through a rangefinding device as opposed to simply guessing or estimating the distance and direction. Additionally, a well designed system will accept that range and bearing digitally, so that the user does not have to risk any data entry errors. What type of feature is being offset?
Users should also consider that applying an offset to a point feature is different than applying an offset to a line or polygon feature. There is ultimately only one offset that can be applied to a point feature. This involves simply translating the point to another location. For example, an offset of 5 meters to the northeast (on a bearing of 053 degrees); it is very simple to visualize moving the location 5 meters to the northeast. (Note, however, that it is possible that the one applied offset was computed by chaining (or adding) multiple offsets together. For example, the combination of 3 meters due north and 4 meters due east would result in a total offset of 5 meters to the northeast on a bearing of 053 degrees.)
On the other hand, with a traverse-like feature such as a line or a polygon, a single offset may not be appropriate. For example, a user may wish to record the shore of a lake by walking around the lake at a constant offset of 5 meters from the shoreline (on the dry side of the shore). In this example the user will not want the same offset direction applied to every position. Instead, it would be much more useful if the GPS unit applied an offset based upon the user's direction of travel. Such an offset might be described as "5 meters to the left, orthogonal to the direction of travel." Additionally, the user may require several offsets to represent different segments of a line feature. In the example above, there will probably be places where the user cannot maintain a constant 5 meters from the shoreline and is forced (perhaps by a big mud puddle) to walk part of the shoreline at a distance of 8 meters. A well designed GPS that utilizes offsets will cater to these differences between point features and line or polygon features. Things to be wary of...
Often the weak link of an offset measuring device is the compass. There are several errors that occur commonly in the field when using any type of compass. The issues below are generally not a serious limitation or problem as long as the user is aware of them and pays attention to the equipment at hand.
When using any type of compass, it is important that you give the compass a little time to settle down at a stable value. Perhaps the most common error occurs when a user lifts the compass and attempts to read a value while the needle is still fluctuating within a few degrees of the correct bearing. Most users don't make such a silly mistake when using a traditional compass that has a clearly visible needle. However, there often is no visible "needle" when holding a device that contains an internal electronic compass. It is best to take your time aiming before you shoot a bearing, and even better if you take a few shots for comparison.
Another potential error can occur if the user pushes a ranging device beyond reasonable limits. For example, suppose you are using a laser that can accurately measure distance to 6000 meters and has a compass that is accurate to 0.5 degrees. Even when you have a wonderfully clear day and a reflective target, the accuracy of your results will still be limited by the compass. Remember that 0.5 degrees of compass error will contribute nearly 1 meter of error when projected to a target only 100 meters away.
Magnetic declination is another issue to consider. Like the two issues mentioned above, it is not a problem as long as you are aware of it. Pay attention to whether the laser is configured to provide bearings with respect to true north or magnetic north. Likewise, be aware of how the GPS system will handle any bearings that are received from a laser. Will they be processed as if they are true or magnetic bearings? There is no right answer. As long as the GPS and the laser are configured so that the magnetic declination will not be applied twice, and the bearing will not be mislabeled, there is no problem. Summary
This month's column addresses only a few of the important things to consider when evaluating the combination of GPS and offsets. To optimize accuracy, consider that your data will have much better integrity if your GPS unit can be attached directly to an electronic measuring device, such as a laser, rather than requiring manual measurement or manual data entry.
To optimize usability, ensure that the systems you consider are practical for the type of data that you collect. If your job is to collect data at point features, such as telephone poles, your requirements will be very different form the person who collects data at line/polygon data, such as street centerlines, curbs, or wetland boundaries. If your job requires the collection of points, lines, and polygons, you should be careful that the system you buy is flexible enough to meet your needs. Next month, we will consider several additional factors that relate directly to how GPS and lasers can be used in the field. 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. If you have a suggestion or request for a future article, please drop a line to Chuck care of EOM, 13791 E. Rice Place, Suite 204, Aurora, CO 80015, fax to 303-690-2522, or send via e-mail: [email protected] Back |
