GPS Mapping

For many years now, a sharpener and pencil have not been enough if you want to make a proper orienteering map. New technologies have significantly influenced the work of cartographers. Since two to three years ago, GPS devices were either not accurate enough or too expensive. But presently, it seems that the era of using GPS technologies in the creation of orienteering maps has fully begun.

The Global Positioning System was originally produced by the US army. It allows for detecting the position of a subject via satellites circling in orbit. Position detection by satellites works on the same principle as detection of an unknown object in mapping – by crossing two or more azimuths. The satellites know each others’ positions; their transmit signals meet in a receiver-unit that you have in your hand; and thus you pinpoint your location.

GPS usage is not necessary for every map: for example, if you are working with a base map made upon evaluation of aerial photos. Since photogrammetry contains plenty of catching features, GPS is therefore redundant. Russian mapmaker Alexander Shirinian comments: "A GPS device is a very progressive instrument. I use a Silva Multinavigator at the early stages of my work; but, if you have a good base-map, it is not that important whether you use it or not.”

The main criteria when deciding whether to work with GPS or not should be effectiveness of the work. Maps for WOC 2005 in Japan are being made with the help of Trimble Pathfinder Pro XR device. The reason for employing this is that the thick cover of cedar forest does not allow for the creation of high-quality photogrammetric materials. Hatori Kazushige, the WOC 2005 GPS specialist, explains: "Working with GPS saves 25 % of the time you would have otherwise spent in field-checking.”

Dressing Skeletons in Cartographic Meat

Mark Sylvester, an Italian mapper, explains: "I use the GPS receiver Trimble GeoExplorer in the initial stage of mapping. I cover the terrain with the receiver, recording anything that looks worthy. I enhance the public photogrammetric base-map with extra data: paths, walls, all kinds of point and line features. At home, I do differential corrections, and import the data as dxf into OCAD, with the base-map as the template. I load over the recorded features from the GPS receiver. Later, I print the base-map and return to the terrain to do fieldwork as usually.”
In contrast, Ed Hicks, a US mapping professional, goes to the forest directly with his laptop connected to GPS: "I am running Ocad in the field, with the GPS signal indicating where I am on the base-map. Accuracy is about 1-2 meters. As my computer, I have been using the VIA-PC wearable, with a sunlight-readable screen.

In present, it is difficult to say which of the two methods will prevail. Currently, it is still two-stage mapping that dominates the trend – creating a bare map-skeleton using GPS, followed by gradual fleshing in with map information in a second survey pass. Still, it is highly probable that the more technically sophisticated (but also more expensive) map-making-while-using-a-laptop-in-the-field will eventually prevail. This method is more straightforward, thus helping to save time.

The main requirement for the use of GPS is accuracy: in the creation of orienteering maps, the accuracy deflection should be within 2 meters, when compared to the actual position. This requirement defines parameters that should by met by GPS technologies used in mapping. Another important factor is to know in which conditions GPS works best and what can influence its accuracy.

Kevin Haywood, from Georgia Orienteering Club in the USA, strongly recommends using an external antenna with a GPS receiver: "Your body blocks satellite reception from behind you. An external antenna mounted at head-height will receive satellite data from a complete 360-degree view.”
Thick branches also block signal reception. In open terrain, signal transmitting grows. The quality of signals transmitted from satellites can also be influence by weather. If the sky is covered with clouds, the accuracy is less. Positioning of satellites can also be detected in advance by computer software, and the time of mapping can be planned according to that information. In order to reach accurate measurement data, you have to have at least four well-positioned satellites available.
The accuracy of GPS positioning can be enhanced by differential corrections. However, the availability of differential corrections differs from country to country. GPS signal often contains errors that can be detected by a public GPS receiver which has been precisely positioned. Such identified deflection allows you to exactly identify the position of a mobile GPS mapping-station.
Differential corrections can be obtained either in real time (an error is correct directly at the moment with a GPS unit) or backwards (using so-called “post-processing”). Hatori Kazushige considers post-processing a key factor in improvement, a factor that enhances the accuracy of logged data.
In the WOC 2005 competition area, there are three different types of differential information available for post-processing, three distinct beacons: Nagoya 50 km (10-sec interval); Daiousaki 100 km (10-sec interval); Kyoto 150 km (5-sec interval). The best accuracy is provided by the Nagoya beacon, and the results from the Kyoto beacon are satisfying as well. If the correction interval is longer than 20 seconds, or it is more distant than 200 km, GPS measurements are not good enough.
Hatori Kazushige assumes: "95% or more logged points, if you measure for 12 seconds, are within less than a 2-meter drift. The results are really satisfying at present.”

Ales Hejna 2004-10-06 (published in O-sport 3/2004)