Post on 02-Jan-2016
USE OF GPS FOR CROP AREA MEASUREMENT
The CIRAD experience
PRESENTATION OUTLINES
GPS System
Area Calculation with GPS
Experiences and lessons learnt
PRESENTATION OUTLINES
GPS System
Area Calculation with GPS
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
(20 200 km)
• Orbit in 11h58min
• Moving user visible constellation
Trilateration
Distance between
satellite and user receptor is computed accorded to time delay
Three satellites are enough to determine a position on the ground
Satellite and
receptor should have synchronized clocks (~1ns)
A fourth satellite is necessary to adjust the receptor clock
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 andDilution of precision
PDOP < 6
Constellation geometry andDilution of precision
PDOP > 6
Dilution Of Precision
Good GDOPPoor GDOP Poor GDOP
How to improve precision? Clear obstacles
Good GDOP (depending of situation) Avoid multipath
Multiple measurements (delay?)
PRESENTATION OUTLINES
GPS System
Area Calculation with GPS
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 CONIQUE LAMBERT 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
GPS System
Area Calculation with GPS
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 without 20° 30°mask mask mask
10
0
Measurements under forest
Lessons learnt
• Measurements are better outside forestUse 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
Manual recording of data
Lessons learnt
• Do we track full plot polygons or limits between plots• Graphics GPS can help on complexplot distribution• Small plot shape errors are difficult to recover over time• Direct GPS/PC connection saves timeand 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 orGoogleInaccuracies
(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 areaprotocol needed to optimize accuracynumerous ways to make errors
•From plot area to (light) GISmust 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 softwareand standardized data format exchange