1. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 1 GPS in o-mapping

2. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 2 How does it work?

3. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 3 The GPS system consists of three pieces. There are the satellites that transmit the position information, there are the ground stations that are used to control the satellites and update the information, and finally there is the receiver that you purchased. It is the receiver that collects data from the satellites and computes its location anywhere in the world based on information it gets from the satellites. There is a popular misconception that a GPS receiver somehow sends information to the satellites but this is not true, it only receives data. How does it work?

4. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 4 The GPS receiver measures its distance from the satellites and uses this information to compute a fix. How can it measure distance? Well it really measures the length of time the signal takes to arrive at your location and then based on knowing that the signal moves at the speed of light it can compute the distance based on the travel time. However, unlike the known sites of the olden days, these sites are moving. The solution to this problem is to have the satellite itself send enough information to calculate its current location relative to your receiver. Now, armed with the satellite location and the distance from the satellite we can expect that we are somewhere on a sphere that is described by the radius (distance) and centered at the satellite location. How does it work?

5. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 5 Operating the GPS The development of GPS started in 1972 by the USA Ministry of Defense. The original purpose was military but the civil use was also taken into account in the beginning. By acquiring the same information from a second satellite we can compute a second sphere that cuts the first one at a plane. Now we know we are somewhere on the circle that is described by the intersection of the two spheres. If we acquire the same information from a third satellite we would notice that the new sphere would intersect the circle at only two points. If we know approximately where we are we can discard one of those points and we are left with our exact fix location in 3D space. Now, what would happen if we were to acquire the information from a fourth satellite? We should expect that it would show us to be at exactly the same point we just computed above.

6. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 6 Satellites The satellites are travelling around the world 20200 kilometers high in carefully controlled orbits at a speed that means they will make a complete orbit twice a day. Each orbit takes 11 hours and 58 minutes, so like the stars they will seem to drift 4 minutes a day. The complete constellation consists of a minimum of 21 active satellites and 3 working spares. Currently there are 27 total satellites in the sky and it is possible that there could be as many as 31 or 32. There are 6 orbits with multiple satellites in each orbit as depicted in the drawing at the top of the page. Each orbit is inclined 55 degree from the equator and thus there are no orbits that go directly over the poles, but certainly a great many orbits can be seen from the poles or anywhere else on the earth. The goal of the system is to always provide at least 4 satellites somewhere in the visible sky. In practice there are usually many more than this, sometimes as many as 12.

7. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 7 Satellites 2 Similiar systems were developed and operated by Sowiet Union. The system is called GLONASS. In 1993 13 satellites were poerated (21 was planned), the height is 19100 meters. After the selected availibility of GPS cancelled in 2000, the importance of Glonass was reduced.

8. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 8

9. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 9 Earth stations There are 5 Eath stations measure the positions of the satellites. The central station is in Colorado Springs. The corrected data forwarded back to the satellites for adjusment.

10. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 10 Users The main components of the GPS devices are the antenna unit and the signal pocessing unit. The antenna unit receives the composite signals of the satellites. The modern GPS device have multi channel capabilities, which means that they are suitable to receive signals of more satellites simultaneously. GPS uses the WGS-84 reference system. If we working on a different projections system we have to transfrom our measures.

11. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 11 GPS receivers Simple receivers, Geodetic receivers, Static measuring systems, Built-in systems (car navigation).

12. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 12 GPS receivers

13. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 13 GPS receivers

14. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 14 GPS rceivers

15. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 15 GPS manufacturers

16. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 16 Map production - 1 Maps are rarely created from scratch by ground survey. Base maps are generally obtained from topographic maps or photogrammetric (PG) plots. Frequently old orienteering maps form the base map. Old orienteering maps: Only as good as the quality of the previous map, distortions can be carried over and quite large errors very difficult to spot. Multiple interpretations of feature size / significance over time

17. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 17 Map production - 2 Photogrammetric plots: Poor in heavy forest as heights are estimated from tree tops so deep valleys are underestimated as trees tend to grow higher to get light. Good delineation of large linear features, smaller ones (especially in forest) will be missed. May have minor contour features that do not reflect real or useable features on the ground. Survey: Ground survey using the base map, then transferred to the final map, usually on OCAD.

18. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 18 Comments, opinions 1 Very good for street/park maps 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. Working with GPS saves 25 % of the time you would have otherwise spent in field-checking. 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.

19. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 19 Comments, opinions 2 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. 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.

20. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 20 Accuracy We must be aware of the many situations in which a GPS may not record data with adequate accuracy for map making. Cloud cover will absorb and reflect satellite signals. Leaf cover will block the view of the satellites. At any one time, satellites may be positioned poorly for optimum reception. To judge the accuracy of your GPS, follow a trail up and back to your start. The two track plots should follow each other precisely. If successive recordings of a trail do not match very closely, do not use the GPS data on your map. In many situations, traditional methods (compass bearings and pacing) may be more accurate than GPS data.

21. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 21 Danger of GPS Use of a GPS, as an advantage in Foot-O competition, is highly unlikely because of the fairness and for the time required to enter coordinates and the requirement to get one's mind completely off the map in order to manipulate controls and read/interpret/plot results. In short, a non GPS equipped Orienteer will likely have the advantage because of the lack of distraction caused by the GPS. Sometimes it is really difficult to fit our GPS measurement and our o-map.

22. O-Map Seminar, Curitiba, Brazil, October-November 2006 Instructors: László Zentai (HUN), Flemming Hjorth Jensen (DEN) 22 So what is GPS good for in o-mapping? If we have no access to base maps GPS can be very useful to make the „skeleton” of the map. It is difficult to utilize the height information (it is provided by the GPS, although the accuracy is limited). We have to take into account the limited accuracy of the cheap GPS receivers, we have to prepare our measurements (be familiar with the system) If we have base map, but it is not good enough, we use the GPS for collecting points and tracks to make our further fieldwork easier, more precise. Even the best base maps can make better with GPS points; but we have to fit the two systems (GPS WGS-84 and the national mapping system) and be familiar with system.