USE OF GPS FOR CROP AREA MEASUREMENT The CIRAD experience.

USE OF GPS FOR CROP AREA MEASUREMENT

The CIRAD experience

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?)


GPS System



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.)


GPS System




• 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



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


Direct GPS import with google earth plu


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


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

USE OF GPS FOR CROP AREA MEASUREMENT The CIRAD experience.

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