THIS PAPER PRESENTS :

1. Projet GCP/INT/903/FRA‘‘Appui au Programme de Renforcementdes Systèmes d’Information et de Statistiques Rurales en Afrique’’TECHNICAL MEETING ON THE USE OF GPS FOR CROP AREA MEASUREMENT IN THE AGRICULTURAL SURVEYS IN AFRICAAddis-Ababa, Ethiopia: 27-28 November 2008

2. USE OF GPS FOR CROP AREA MEASUREMENT: DOCUMENT REVIEW AND OVERVIEW OF FIELD EXPERIENCES CONDUCTED BY FAObyPaul N’GOMA-KIMBATSAStatisticianStatistics DivisionFAO, Rome

3. THIS PAPER PRESENTS  : • I. GENERAL BACKGROUND ON THE GPS SYSTEM • II. OVERVIEW OF FIELD EXPERIENCES

4. I. GENERAL BACKGROUND ON THE GPS SYSTEM • GPS : What is GPS ? • Accuracy of the GPS • Navigation Satellite Systems • Differential GPS Technique (DGPS) • Suppliers and Equipments of the GPS

5. I.1. GPS : WHAT IS GPS ? • The Global Positioning System (GPS) is a Global Navigation Satellite System (GNSS) developed and controlled by the United States Department of Defense in the year 1970. • The GPS is made up of three parts: • Satellites orbiting the earth. • Control and monitoring Stations on earth. • GPS receivers owned by users.

6. I.1. GPS : WHAT IS GPS ? • GPS satellites broadcast signals from space that are picked up and identified by GPS receivers. • 4 satellites are required to provide 3 dimensional location of X, Y, Z (Latitude, Longitude and Altitude) and Time.Once the user's position has been determined, the GPS can calculate other information, such as area.

7. I.1. GPS : WHAT IS GPS ? • Since 1994, 24 satellites are orbiting the earth about 12,000 miles above us and make 2 complete orbits every 24 hours. • GPS satellites continuously transmit digital radio signals.

8. I.1. GPS : WHAT IS GPS ?

9. I.2. ACCURACY OF THE GPS • The accuracy of a GPS Receiver means Absolute Error of the position measured in relation with a known position. • Common Factors affecting the accuracy of GPS: • GPS Technique employed (Autonomous, WADGPS, DGPS, RTK, etc.) • Surrounding conditions (satellite visibility and multipath) • Number of satellites in view • Satellite Geometry • Distance from Reference Receiver(s) (non-autonomous GPS i.e.: WADGPS, DGPS, RTK) • Ionospheric conditions • Orbital errors • Quality of GPS receiver

10. I.3. NAVIGATION SATELLITE SYSTEMS • The various satellite navigation systems available are: • WAAS ‘‘Wide Area Augmentation System USA’’ (25 ground reference station, 2 master stations and 2 geostationary satellites) • EGNOS ‘‘European Geostationary Navigation Overlay Service’’ (Accuracy 1-5 meters) (3 geostationary satellites with the network of ground reference stations) and the upcoming European GALILEO • Russian GLONASS ‘‘Global Orbiting Navigation Satellite System’’ • MSAS ‘‘Japanese Multi-Functional Satellite Augmentation System’’ • COMPASS Navigation System of China • IRNSS of India

11. I.3. NAVIGATION SATELLITE SYSTEMS WAAS “Wide Area Augmentation System” provides a position accuracy of 3 meters or better, at least 95% of the time.

12. I.4. DIFFERENTIAL GPS TECHNIQUE (DGPS) • Differential Global Positioning System provides differential corrections to a GPS receiver in order to improve the accuracy to within 1-3 meters. • The principle of differential method (DGPS) is the following: • DGPS works using a network of stationary GPS receivers. • The difference between their predefined position and the position as calculated by the signals from satellites gives the error factor. • This error component is then transmitted as a FM signal for the local GPS receivers, enabling them to apply the necessary correction to their readings.

13. I.4. DIFFERENTIAL GPS TECHNIQUE (DGPS)

14. I.5. SUPPLIERS AND EQUIPMENTS OF THE GPS • The major manufacturers of the GPS are: GARMIN, MAGELLAN, TRIMBLE, FORTUNA, THALES, LOWRANCE, SATCON, NAVMAN, TOMTOM, HOLUX, HAICOM, NAVIGON, MIO, BLAUPUNKT, NAVIGON, SONY, NAVMAN, BECKER, VIAMICHELIN, etc. • There are 3 categories of GPS receivers : Avionics, Marine and Land, also for auto. The models of GPS receivers with better price/performance ratio most commonly used for calculating the area of plots are:

15. I.5. SUPPLIERS AND EQUIPMENTS OF THE GPS

16. I.5. SUPPLIERS AND EQUIPMENTS OF THE GPS • GPS Garmin 76 : 179 € TTC • GPS Garmin eTrex Legend : 205 € TTC • GPS Garmin eTrex Vista : 235 € TTC • Garmin GPSMAP 76 : 273 € TTC

17. I.5. SUPPLIERS AND EQUIPMENTS OF THE GPS • GPS Garmin 12 : 63 € TTC • GPS Magellan eXplorist 400 : 205 € TTC • GPSMART Fortune BT - Bluetooth Autonome: 219 € TTC • GPS THALES et GPS SATCON

18. II. OVERVIEW OF FIELD EXPERIENCES • II. 1. UGANDA • II. 2. MOZAMBIQUE • II. 3. USDA-FOREST • II. 4. POLAND UNIVERSITY • II. 5. CIRAD-FRANCE • II. 5. FAO (Rome, Cameroon, Niger, Senegal, Madagascar)

19. II.1. UGANDA • Scope/Coverage: Pilot Census of Agriculture (March to July 2003). Test of 15 small and medium scale farmers. • Materials used: GPS GARMIN 12 powered by 4 AA rechargeable or long life batteries. • Conditions of the experience: Hilly terrain and tree canopy cover dense.

20. II.1. UGANDA • Methodological approach: The holdings in the enumeration areas (EA) were randomly divided into three groups/strata each with five holdings. 4 approaches to area estimation for different strata of holdings as shown below: • Holders’/respondents’ eye estimates of parcel and crop plot area (5 selected holdings in Group I). • Enumerators’ eye estimates of plot area (10 selected holdings Groups II & III). • Measuring using compass and measuring tapes (5 selected holdings Group I). • Measuring by use of GPS equipment (all 15 holdings).

21. II.1. UGANDA • Results : Table 1: Comparison between different equipment and repeated measures by one equipment (Square Meters)

22. II.1. UGANDA • Results : Table 2: Repeated Plot Area Measurements by One GPS (January 2003) (m2) Measurement by tape and compass gave 483 square meters.

23. II.1. UGANDA • Conclusion : • The area measurements by the GPS equipment and those by the Compass/Tape are very close. • The GPS equipment is much faster and that costs are fairly comparable. • There was therefore variability between the different equipments and by the same equipment used repeatedly. • The eye-estimates by both the enumerators and the farmers/holders are inconsistent. • More study is required on the variability and consistency of the equipment, especially on steep slopes and under tree and cloud cover. • Shadows of trees, buildings etc should be avoided while using the GPS equipment in the field.

24. II.2. MOZAMBIQUE • Scope/Coverage: Survey ‘‘Trabalho Inquérito Agrícola TIA’’ TIA 2002. • Materials used: GPS GARMIN eTrex Venture. • Methodological approach: Measuring of 620 holdings selected by using of GPS and Compass/Tape.

25. II.2. MOZAMBIQUE • Conclusion : • Using GPS for the small parcels is not appropriate: Accuracy (5-15 meters). • The accuracy increases with the size of the parcel. • The time taken to use the GPS for measurement was always much less than the Compass/Tape (less 60%). • GPS can also be used to geo-reference holdings (x/y coordinates)

26. II.3. USDA-FOREST • Objective: Test of 4 GPS Receivers. Dense cover - Forest (Site of Clackamas) and No cover (Station of Estacada). • Materials used: GARMIN GPS III Plus, GARMIN eTrex, MAGELLAN GPS 2000XL, MAGELLAN Blazer.

27. II.3. USDA-FOREST • Conditions of the experience: Dense cover (Forest) and No cover. • Methodological approach: All experiences made without SA (SA ‘‘Selective Availability’’ off). • Calculation for Accuracy: RMS : Accuracy dRMS : Error

28. II.3. USDA-FOREST • Results:

29. II.3. USDA-FOREST • Conclusion : • Accuracy acceptable on the No Cover Site. • Diminution significant of the accuracy on the Dense Cover (Forest), particularly with GARMIN GPS III. • Necessity to increase the number of measurements on the Dense Cover (Forest).

30. II.4. POLAND UNIVERSITY • Objective: Validation Methods for Measurement of land parcels areas. • Materials used: 2 GARMIN GPSMap 76S, 4 GPS THALES, 4 GPS SATCON. • Conditions of the experience: Good (No cover) – Bad (Forest).

31. II.4. POLAND UNIVERSITY • Methodological approach: • 36 land parcels are measured (with different size: S “Small 0.3-0.5 ha”, M “Medium 0.8-1.2 ha”, L “Large 2.4-4 ha”): 18 parcels for each experiment “Good” and “Bad”. • 3672 field measurements were made independently by each of 4 teams during 6 days. • Test skilled and unskilled operators. • Accuracy estimation was based on point position error using following formulas:

32. II.4. POLAND UNIVERSITY • Methodological approach:

33. II.4. POLAND UNIVERSITY • Results :

34. II.4. POLAND UNIVERSITY • Conclusion : • Error of parcel area and the Standard-Deviation (m) increase with size. • Storing all data in GIS data base is suggested. • 2 kind of parcel size seems to be enough (instead of 3 kinds). • It is not necessary to test skilled and un skilled operators (the obtained results are similar for both, but unskilled operator must be more detailed trained).

35. II.5. CIRAD-FRANCE (Specialist Experience) • Objective: Test of the accuracy of GPS Garmin 12. • Materials used: GPS GARMIN 12. • Methodological approach: • 85 measurements were made on 33 land parcels. • 1 measurement on the small parcel (< 1 ha) and a lot of measurements on medium parcels.

36. II.5. CIRAD-FRANCE (Specialist Experience) • Results: • Conclusion: • It is not necessary to multiply the number of measurements to obtain a good accuracy.

37. II.6. FAO • ROME • CAMEROON • NIGER • SENEGAL • MADAGASCAR

38. II.6. FAO (Suburb of Rome) • Scope/Coverage: Test of Holdings in the Suburb of Rome, Italy 2003. • Materials used: GPS GARMIN eTrex. • Conditions of the experience: Good visibility. • Methodological approach: Field measurements were made independently on 2 kinds of plots (Small and Large) by using GPS and Triangulation method.

39. II.6. FAO (Suburb of Rome) • Results: Small Plot • Area measurements with GPS : • Mean: 153,25 m² • CV: 26% - Triangulation : Area = 150 m²

40. II.6. FAO (Suburb of Rome) • Results: Large Plot • Area measurements with GPS : • Mean: 7344 m² • CV: 11% - Triangulation : Area = 7444 m²

41. II.6. FAO (Suburb of Rome) • Conclusion : • GPS : Easy to use • GPS : Saving the time of measurement • GPS : Application is simple, thus not much training is needed and possibility to make errors is reduced. • GPS : Error in case of smaller plots is very large. • The combination of two methods is likely to be the best approach: Measuring the small plots with compass bearing (or some other method) and large plots with GPS. • Closed environments, such as tree cover, metallic fences, walls, etc. may decrease GPS precision.

42. II.6. FAO (CAMEROON) • Scope/Coverage: The pilot experience was conducted in the region Mfou : Equatorial zone with humid forest. • Materials used: GPS GARMIN 72, GPS GARMIN 60 et GPS Magellan Explorist 400. • Conditions of the experience: Bad visibility - Tree canopy cover dense.

43. II.6. FAO (CAMEROON) • Methodological approach: • 2 supervisors and 9 enumerators were trained and took part in the pilot experience, with FAO Technical Assistance. • 249 Field measurements were made independently during 4 days by using GPS and traditional method (Compass/Tape). • For each plot, 2 estimations were registered on the questionnaire: one done by the farmer and a second by the enumerator. • Measurements were done by using the 3 models of GPS and by traditional method (Compass/Tape).

44. II.6. FAO (CAMEROON) • Conclusion: • Measurement of crop area using GPS is easier and takes less time than traditional method (Compass/Tape). • It was not necessary to repeat the measurements of a plot to improve the accuracy. • No good accuracy by using GPS for the measurement of small plots area. • Weak accuracy of measurements with GPS when the distance between two points were close to each other (less than a dozen metres).

45. II.6. FAO (CAMEROON) • Conclusion: • GPS Garmin 60 is the easiest to be manipulated by the enumerators. • GPS Magellan proved to be difficult in manipulation by majority of enumerators; • There were a high number of errors committed by enumerators by using Garmin 72 due to the need of recording each sides of the plot. • It is important to choose a model of GPS with long live batteries. • In forest covered area, it is important to light the GPS in an uncovered area before entering the forest. This will allow the GPS to connect to the satellites.

46. II.6. FAO (NIGER) • Scope/Coverage: The pilot experience was conducted in 2 regions (Dosso and Tillaberi) : Zone Sahelo Sudanese. • Materials used: GPS GARMIN eTrex Venture, GPS GARMIN 72, GPS GARMIN 60, GPS GARMIN 12 et GPS Magellan Explorist 400. • Conditions of the experience: Good visibility - No cover.

47. II.6. FAO (NIGER) • Methodological approach: • 10 enumerators, 2 controllers and 2 supervisor were trained and took part in the pilot experience, with FAO Technical Assistance. • Measurements of the plots of 40 holdings selected by using the 5 models of GPS and by traditional method (Compass/Tape). • For each plot, 2 estimations were registered on the questionnaire: one done by the farmer and a second by the enumerator.

48. II.6. FAO (NIGER) • Conclusion: • Measurement of crop area using GPS (5 mn) takes less time than traditional method (Compass/Tape) (15 mn). • There isn’t the significant difference on the result obtained between the GPS and the traditional method (Compass/Tape). • Good accuracy by using GPS Magellan Explorist 400 for the measurement of large plots area. • Bad eye-estimation by the farmer/holder.

49. II.6. FAO (SENEGAL) • Scope/Coverage: The pilot experience was conducted in 2 departments (Thies and Tivaouane). • Materials used: GPS GARMIN 72, GPS GARMIN 60 et GPS Magellan Explorist 400. • Conditions of the experience: Good visibility - No cover.

50. II.6. FAO (SENEGAL) • Methodological approach: • 2 supervisors, 2 controllers and 6 enumerators were trained and took part in the pilot experience, with FAO Technical Assistance. • Field measurements were made independently and with repetition (3 times) on about 48 plots (small and large) during 4 days by using 3 kinds of GPS and traditional method (Compass/Tape).