In Review: TDS Locator

February 1, 2005
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An effective tool to aid surveyors in navigating to the approximate location of a monument.

Tripod Data Systems
Corvallis, Oregon
Suggested List Price: $119*

*TDS Locator requires a Recon starting at $1,799. A bundled package including software, GPS card and Extended CF-Cap is $299.

Some of the developments that have had a major impact on the ways surveyors work include logarithms in the 1600s [1]; the first mechanical calculator capable of adding, subtracting, multiplying and dividing in the early 1800s [2]; the first 10-key calculator in 1901 [3]; the first shirt pocket-sized scientific calculator (HP35) in 1972 [4]; and the microchip in 1971 [5]. The latter led to the development of the electronic total station, and data collectors and GPS receivers as we know them today.

As each of these major technological breakthroughs surfaced, the fine-tuning that followed produced many spinoffs. And the surveying profession has been there to put them to good use. One such spinoff of GPS technology is the development of small low-cost receivers that can be carried in a shirt or coat pocket.

These small handheld GPS devices have become quite popular for general-purpose navigation. Relatively inexpensive units are available in a variety of forms and many are designed to work with a Pocket PC. Recognizing the challenges surveyors face in locating property corners on large tracks or in remote areas, Tripod Data Systems (TDS) has developed TDS Locator, a program to help locate corners and other items of interest through the use of a low-cost GPS receiver that will work with the TDS Recon, its rugged handheld computer.

Figure 1. The route entry/exit screen.

Locating Points

All that is needed to use TDS Locator is a TDS Recon, a GPS receiver that puts out an NMEA (National Marine Electronics Association) data stream, TDS Locator software, and information on what you are looking for. This information can be a latitude and longitude position, or it can be a series of bearing and distances describing a traverse, boundary or any series of points with known bearing-distance relationships.

Points that are to be located by latitude and longitude can be entered directly, or imported from an ASCII file in the point name, latitude, longitude, elevation, description format (waypoints). Points can also be entered by azimuths and distances, a valuable format for surveyors. (Bearings can also be used, but all distances are displayed in azimuths.) Points entered in this manner do not have a specific starting location and are called routes. (See Figure 1 below.) Once any point in a route is located in the field, the route is localized (assigning a GPS ground coordinate to the found corner) by standing on the point, navigating to the Localize screen shown in Figure 2 below and pressing the "Localize" button. This assigns a latitude and longitude coordinate to each point in the route. Once a route is localized, a graphical display of the route is shown on the screen along with an indicator showing the user's position. Any point defined in the route or any waypoint can be selected as a target. More than one route can exist in a project, but only one route or target can be active at a time. When points of interest are located in the field, their positions can be recorded and are stored as waypoints. Any waypoint can be used as a reference point, which provides an additional aid in navigating the site.

As one moves across a project after identifying a target point, three indicators assist in locating the target. One is a display of the distance to the target, the second is an arrow pointing in the direction of the selected target, and the third is a compass. Another indicator shows the distance to a reference point, if one has been identified. Direction indicators are relative to the direction the user is moving, and as such are not active when one is standing still. These direction indicators appear in the circles shown in the upper right corner of Figure 3 on the right.

If one needs to change the current target to a different point in the route, two handy icons in the lower left corner of the screen, as seen in Figures 3 and 4 on the right, make it easy to cycle through points in a route, either forward or backward, to change the target point.

Figure 2. Localizing a point.

Loading Images

Surveyors use aerial photos in a variety of ways, so it is quite natural to want to use one when searching for corners or other items in the field. TDS Locator has the ability to load a geo-referenced image file (an image that is referenced to the real world by position and scale) as a background, which will aid in locating a target. Currently, TDS Locator can only load an image file with the JPEG format. This format was chosen because it is readily available for free from the Microsoft website [6]. The ones I downloaded had a resolution of 1 meter per pixel and were taken in 1993. Dates on others that I examined on the Microsoft website ranged from 1988 to 2003. Most had a resolution of 1 meter per pixel, but some were as detailed as 0.25 meters per pixel.

If an image file is loaded, the current location, route, reference and target points are shown over the image. It is easy to select an item from the image and establish a waypoint at the object's location. This is accomplished by selecting the pushpin at the bottom of the image and then clicking on the object's image. This is a handy aid in using an image to locate points of interest, especially those where it is difficult or impossible to pre-establish a set of coordinates.

I searched the Internet for other sources of images and found some that were more recent. While they were not free, the cost was reasonable. (A 1-meter resolution, 500 x 500 [pixel] meter image, or 2.25 square mile image, cost between $4.95 and $6.95.) I downloaded a couple of them but found that I couldn't use them since the accompanying world file7 was not in the proper format.

I talked with Rich Williams, business development manager of Virginia Resource Mapping, Sterling, Va., about the potential for surveyors to get digital images with a matching world file from an aerial firm. I was told that geo-referenced images are commonly delivered for use in a GIS system and can be developed for any existing photo if control points with known coordinates are identifiable on the photo.

Figure 3. Main Locator screen zoomed in for a closer view of a target point.

Answering Surveyors' Questions

I showed the TDS Locator unit to a number of surveyors and received significant interest and some questions. One question was "What do I do if my bearings are not true bearings?" A feature of TDS Locator is the ability to rotate a route by a number of degrees. If one is unaware of the rotation needed prior to going to the field, the amount can be determined in the field in a couple of ways. One is trial and error using a loaded image as a reference, trying different values until the route appears to be aligned properly on the image. I used this method to rotate my lot to line up with a row of pine trees. Another way-and a more direct procedure-is to locate a second corner and record its position. The azimuth between the localized point and the second corner can be measured graphically and this value compared with the measured azimuth between the same two corners.

The accuracy of the measurement depends on the ability to pick the points from the screen. This can be difficult if the distance between the points is great, since both points must be visible on the graphical display. I would like the ability to localize on a second point and have TDS Locator calculate the proper rotation.

Another question I was asked was "Can I scan an old aerial photo and use it to find an old fence corner or something of historical significance, such as an old building site that no longer shows on recent images?" While this is theoretically possible, in order to register the photo to the ground coordinates, a companion world file must be created. As stated earlier, an aerial firm can do this if there are two or more control points identifiable on the photo. Since it may be more timely to do this in the office using one's own scanner, I looked for a procedure on how to create a world file. I found a couple of references [8] on how to do this but I never tested them.

The ability to locate a lost corner depends on the accuracy of the location provided by GPS. Two and a half meters to three meters at 95 percent of the time is typical of low-cost receivers and if a WAAS (Wide Area Augmentation System) signal is available. While a WAAS signal is theoretically available in the entire United States, according to the literature I read, there are dead spots. I would like to see an indicator on the screen to tell me if I am receiving a WAAS signal.

I talked to one surveyor who had used a similar type of receiver to record the positions of a number of monuments in Baltimore County, Md. He then compared these with the published values. His results were consistent with the 2.5 to 3 meter accuracy published when a WAAS signal is being received.

Figure 4. Main Locator screen showing an entire property.

Recording Positions

TDS Locator has the ability to track the user's position on the screen as he is moving across the landscape or to map the position being received as one is standing still. This shows as a series of small dashes and makes a random pattern as the position is being mapped.

If a user wants to sit on a position for a few minutes, the true position would be somewhere near the center of the tracking pattern that is produced. By watching the screen and recording a position when the position indicator is near the center, a more accurate position will be recorded.

I keyed in my lot description, positioned myself over one corner and localized my lot to the GPS coordinate I received at that location. I then navigated to my other corners in order to locate them. As shown in Figure 3, the distance to the target point is displayed in the upper left corner of the display. It narrowed the area I needed to search with a magnetic locator.

I tested this product using the Holux 270 CF GPS card. This is a popular receiver used in a typical street map program. Other receivers may attach via the serial port. A low-cost receiver such as the Holux will provide a lot or a utility. Whether one already owns a Recon or not, a modest investment will provide significant capability.

Future Improvements

This is the first release of TDS Locator, and as such is the starting point for a much improved and stronger product in the future. Current developments in GPS technology will soon result in the ability for low-cost receivers to provide greater accuracy. Some folks at NGS predict that this could be within 10 to 20 centimeters within five to 10 years. When I remember that in the early days of GPS surveyors spent hours recording data on a given position, then post-processed that in order to get a usable position, it is not difficult to accept this prediction.

According to Eric Hall, product manager for TDS, the company already has a long list of additional features to be considered for future releases, such as plans to expand the acceptable image and world file formats. When I recall the features in the first version of Survey Pro and compare that with the current product, I have great expectations for the future of this product as TDS incorporates suggestions received from early uses of the product.

TDS Locator is an effective tool for the surveyor to use for navigating to the approximate location of a monument, and thus provide confidence that he is near enough to look for reference marks, or to use a magnetic locator. In the case of a large property where general reconnaissance is needed in order to develop a proposal, TDS Locator allows one to walk the boundary in order to get an idea of the existing boundary evidence, observe possible encroachments, or to cruise the interior for wetland boundaries, clearing requirements, obstructions or other important elements affecting a project.

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