USE OF GPS FOR CROP AREA MEASUREMENT The CIRAD experience

PRESENTATION OUTLINES u GPS System u Area Calculation with GPS u Experiences and lessons learnt

PRESENTATION OUTLINES u GPS System u Area Calculation with GPS u Experiences and lessons learnt

GPS Today  Initially developed by USA DoD Objective : localization on earth with 15m precision  Two precision levels Military, with encryption Civilian : lower precision (~100 m)  Since May 2000 Full accuracy to civilian receptor Precision ~10m

GNSS Tomorrow  Other initiatives GLONASS (Russia) -> 2011 GALILEO (Europe) -> 2013 COMPASS (China) India, Japan, etc.  User receptor will use all systems Increased precision (2013 -> ~2m) Faster start phase  GNSS : Global Navigation System Services

GPS system organization Spatial segment Satellites Control segment The “brain” of the GPS- owned, operated, and controlled by the U.S. Government User segment Radio receptor Clock Calculator

THE GPS SPACE SEGMENT 24 satellites Very high altitude ( km) Orbit in 11h58min Moving user visible constellation

Trilateration u Distance between satellite and user receptor is computed accorded to time delay u Three satellites are enough to determine a position on the ground

u Satellite and receptor should have synchronized clocks (~1ns) u A fourth satellite is necessary to adjust the receptor clock u Fifth and following satellites improve precision Trilateration

Spatial segment error sources  Signal propagation through atmosphere  Clocks inaccuracy  Satellite position inaccuracy  Constellation geometry How to correct?  Differential correction  Time of measurement

Constellation geometry and Dilution of precision PDOP < 6

Constellation geometry and Dilution of precision PDOP > 6

Dilution Of Precision Good GDOPPoor GDOP  How to improve precision?  Clear obstacles  Good GDOP (depending of situation)  Avoid multipath  Multiple measurements (delay?)

PRESENTATION OUTLINES u GPS System u Area Calculation with GPS u Experiences and lessons learnt

Overview of the projection problem  Many representations of the 3D World (WGS 84, INT 1909, different DATUMS)  Leading to different measurements of longitude and latitude 3D World to 2D Map m x y

How to project Origine Tangente Isomètre Origine On a cone Or a cylinder

Different projection systems MERCATOR DIRECTE MERCATOR TRANSVERSE ALBERS CONIQUELAMBERT CONIQUE

Long/lat map Projection « plate carrée » : x=, y=  (ne conserve ni les surfaces, ni les angles)

Conform projection map Projection de Mercator : x=, y= tan(  /2 +  /4) (conservation des angles, distorsion des surfaces)

Equivalent projection map

Distortion in a UTM zone All circles have the same ground area

Other problems Manual coordinate report Units confusion (DD, DM, DMS) Orientation confusion (W/E N/S and +/-) Wrong GPS setup Wrong coordinate transformation setup  Need of assessment data (control points, visualization in GIS, etc.)

PRESENTATION OUTLINES u GPS System u Area Calculation with GPS u Experiences and lessons learnt

Experiences and lessons learnt Measurements under forest Soil plot assessment Plot area measurements Manual recording of data GIS and virtual globe link

Measurements under forest DOP without20°30° maskmaskmask 10 0

Measurements under forest Lessons learnt Measurements are better outside forest Use of external antenna on a mast (essential with dense canopy) DOP varies largely with time

Soil assessment in Mali Linked with GIS

Plot area measurements Several research works in Africa, Vietnam… Lessons learnt Easy to share with untrained people Recorded plot features (shape, area, localization) and attributes recorded on GPS. Data available for future use (no need to measure again an unchanged plot) Sometimes farmer suspicions

Manual recording of data Manual report

Data import, topology to create again Data reformat, reproject… Import data into GIS Manual recording of data

Lessons learnt Do we track full plot polygons or limits between plots Graphics GPS can help on complex plot distribution Small plot shape errors are difficult to recover over time Direct GPS/PC connection saves time and avoids errors

Link with GIS and virtual globes Example from Madagascar (TAFA) With google earth Low resolution and high resolution images

Link with GIS and virtual globes Direct GPS import with google earth plu

Link with GIS and virtual globes GPS or Google Inaccuracies (20m)

Lessons learnt Easy to implement Cost effective Easy to share between people Easy to use More and more high resolution images Limits: No database, No area calculation, First internet connection to download images Link with GIS and virtual globes

Summary GPS system very efficient, handheld GPS precision should increase From GPS to plot area protocol needed to optimize accuracy numerous ways to make errors From plot area to (light) GIS must be more fluent

Conclusions / Perspectives FAO proposed manual is essential It should lead to GIS at short/medium term It could include (or recommend) simple software and standardized data format exchange