By Chuck Gilbert

This article explores the issues associated with GPS data collection. This column explores the benefits provided by various GPS receiver features on today's market. Issues commonly encountered in differential GPS data capture are examined from the user's perspective.

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. The ability to apply offsets to features is an enhancement that has recently appeared in GPS-based data collection systems. 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 key- board. 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 north- east. (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 off- sets 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 reason- able 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 conf1gured 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 off- sets. 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.

By Chuck Gilbert

The GPS Consumer Series is a column that explores the issues associated with GPS data collection. This column explores the benefits provided by various GPS receiver features on today's market issues commonly encountered in differential GPS data capture are examined from the user's perspective.

Introduction:

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 side-bar for a brief recap of offsets and how they apply to GPS data collection systems.)

• 20 meters +/- 0.05m, +/- 0.10m, +/- 0.20m, +/- 0.35m
• 50 meters +/- 0.10m, +/- 0.20m, +/- 0.20m, +/- 0.35m
• 100 meters +/- 0.20m, +/- 0.35m, +/- 0.90m, +/- 1.75m
• 300 meters +/- 0.55m, +/- 1.05m, +/- 2.65m, +/- 5.25m
• 800 meters +/- 1.40m, +/- 2.80m, +/- 7.00m, +/- 14.00m

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 beatings 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 article addresses a few more of the issues to consider when evaluating the combination of GPS and laser rangefinders. The major points are summarized below:

1. If your application requires elevation data, make certain that the system you purchase can accommodate that need.

2. 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.

3. 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.

By Chuck Gilbert

The GPS Consumer Series is a column that explores the issues associated with GPS data collection. This column explores the benefits provided by various GPS receiver features on today's market. Issues commonly encountered in differential GPS data capture are examined from the user 's perspective.

Introduction

This month's column is the third of a series discussing the use of laser rangefinders with GPS. The use of laser rangefinders and GPS is a very powerful combination indeed. Together these tools allow the user to collect attribute and location data very efficiently in situations that would otherwise be very difficult or time consuming.

A typical GPS receiver will compute the location of its antenna. In situations where it is difficult (or impossible) to place the antenna directly on the object to be mapped, a laser rangefinder can provide an offset from the antenna to the object of interest. A well designed GPS system will be able to accept this offset electronically and 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 side-bar below for a brief recap of offsets and how they apply to GPS data collection systems.) Last month, this column discussed a few of the more common error sources that are unique to the GPS/rangefinder combination. This month's column addresses additional aspects of combining GPS and laser range- finders. Depending upon your application, some of these aspects will be very important, and others may be irrelevant.

Constant offsets are good

Users should 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. However, 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 system 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 system that utilizes off- sets will cater to these differences between point features and line or polygon features.

Some GPS systems cater to a special kind of point data collection, using a technique, known as "quickmarking" or as "doing a windscreen survey." (The quickmark technique was described in detail in a previous article, January 1995 issue.) This involves collecting point data while driving or flying non-stop either over or near the objects of interest. If it is convenient to pass directly over the top of the object to be mapped, offsets are not required. However, in applications where it is not convenient to pass directly over the top of the object to be mapped, an offset is a valuable piece of information. For example, suppose you wish to map power poles that are offset from the shoulder of the road by 12 meters. Attempting to drive directly over the poles could have tragic results, so most users choose to drive past the poles while remaining on the road. Ideally the GPS system will allow the user to specify a constant offset to the left or right of their direction of travel. Note also that a good system will allow the user to pre-define different constant offsets for line features as opposed to area features or quickmarked point features.

Taking full advantage of the laser rangefinder

Many laser rangefinders on the market are programmed with the ability to compute other information such as pole height, span lengths, or tree volume. Therefore the owner of a laser rangefinder may wish to take advantage of these other laser features in addition to the laser ranging capability. Ideally the GPS system will always handle the information that comes from the laser in an appropriate manner. For example, if the laser is supplying an offset distance and direction to a feature then that information should be stored in such a manner that it can be applied to the GPS position. On the other hand, if the laser is being used not for an offset, but rather to compute and output the volume of a particular tree it is likely that the user would prefer to store this wood volume computation as an attribute of the tree (not as an offset). A well designed GPS system will allow the user to route the data from the laser to the appropriate field so that the data is stored and used correctly.

Give me some feedback!

When collecting data in the field, it is important to be able to see what values are being computed and stored. Make certain that in any combined rangefinder/GPS system, the GPS is able to display to the user at any time what offset values have been stored for what features. Ideally, the user will have continuous visual and audio feedback on both the laser and the GPS as to what data has been sent by the laser and received by the GPS.

If at first you don't succeed, manipulate the data

The users should have full access at all times to edit the values that have already been received from the laser and stored. Any system that will not allow users to edit their data is seriously flawed. If editing capability is present, note whether the data can be edited only in the field, only in the office after data download to a PC, or both. Ideally, editing should be allowed in both phases of the operation. Field editing is very important, as the user in the field is often the only one who knows the correct value. Office editing is also important, as the user in the office may be viewing the data graphically and able to discover errors that the field user was not able to see.

When evaluating a GPS systems, look for the ability to add offsets later, even if no offsets were stored when the feature was collected. Similar to the editing of offset data, it is best if an offset can be created or added both in the field or later in the office. The ability to add off-set data after the fact can allow some creative options in the post-processing of your data. For example, if the field user drove, one time, down the centerline of a road; given various width attributes, the various lane boundaries, the road edges, and a variety of other parallel features could be easily approximated later in the office.

Versatility in output

After an offset has been stored, how can it be expressed or used? Make certain that the offset data is utilized in all parts of the post-processing software. For example, when you view the features graphically in the form of a map, are the offsets applied? That is, do the features appear at the GPS locations or at their offset locations? Additionally, can the actual offset values be viewed via a simple query? Independent of the display, consider also whether the offset data is utilized by the plotting routines. and the routines that export the data to a GIS or CAD interchange format. A complete system will also allow the user to obtain a textual printout of the offset coordinates. For all of the modules mentioned above, you should also have the choice to view, output, export, display, plot, or print your GPS positions either with or without the offsets having been applied.

Offset from what?

It is important to note that the offset to the feature will be measured from the optical center of the laser rangefinder, not from the GPS antenna. However, after it has been measured, the offset will be applied to the GPS antenna's location. If the laser and the GPS measurements were not made at the same location, you will introduce an error. Consider that for high accuracy work you will need a convenient way to hold the GPS antenna and the laser in the same position. (Remember that placing the laser on top of the antenna will block the GPS satellites and may degrade or eliminate your GPS position.) A well designed system will provide a means of mounting the laser and GPS antenna together. Fortunately, it is not required that both the GPS and laser measurements be taken simultaneously. It is more efficient to take both measurements at the same time, however, the user could take one measurement then the other.

Summary

It is important to examine any system from the beginning to the end. That is, look at the entire process, from the moment the first measurement is taken, to the end product; be it a scaled plot, a textual list, or a file that will be imported into your GIS or CAD system. Try to ensure that in all steps along the way there are no missing pieces that you may have to engineer on your own.