In: Wagner W., Szekely, B. (eds.): ISPRS TC VII Symposium - 100 Years ISPRS, Vienna, Austria, July 5-7, 2010, IAPRS, Vol. XXXVIII, Part 7B
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The metal frames for attaching the sensors were comprised of
three parts, one was inserted into the soil and the other two
parts were erected above ground, one fitting inside the other to
reach the 2m height. This enabled the above ground sections
with sensors to be detached from the lower frame for relocation
to other sample locations leaving the base poles in the ground
throughout the crop season, out of the way of agricultural field
equipment, such as tractors. This design provided the capability
to deploy a large number of base poles inserted in the sample
locations defined based on four different sampling strategies
(Figure 5). This provided the flexibility to change sensor unit
locations to accommodate the various sampling strategies
without the need to install mounting points when sensors were
moved.
The cost of each sensor unit was estimated to be A$1687 (in
2009 dollars).
Ninety sample positions were identified that encompassed the
four sampling strategies in a wheat paddock in Inverleigh,
Victoria, Australia (144° 2’ 30” E and 38° 8’ 10” S, Figure 5).
Rapid static and Real Time Kinematic Global Positioning
System (RTK GPS) surveys were undertaken to establish the
position of the sample points to a x-y accuracy of +/-2cm.
Positions were in GDA94/MGA94. The four sampling strategies
were devised to facilitate the deployment and operation of 20-
25 sensor units at a time. Base poles were inserted in these
ninety locations.
In the first phase of the experiment in 2009, 14 sensor units
were operational and data were recorded from systematic
pattern locations, during the winter crop season from July to
December.
DATA COLLECTION AND ANALYSIS
Wireless remote sensing data collection was observed and
monitored through MoteView Graphical User Interface (GUI).
Sensor nodes were tested for running in two different power
modes for understanding the data relay efficiency and battery
consumption in these configurations.
In this first phase of the experiment, 14 wireless sensors were
introduced during different periods of the crop season, once
they were assembled and tested. During this stage, priority was
given to calibration of the sensors and analysis of the data
quality in comparison with other hyperspectral sensors.
Ground-based hyperspectral data was collected using ASD
FieldSpec® spectroradiometer (ASD, Inc., CO, USA) from the
same locations as those of the wireless sensor units. The ASD
data was integrated based on the bandwidth and central
wavelength corresponding to that of WSN and two data sets
were compared (Figure 6).
144°2'40"E
rfin
144-2'50-E
Node
203
204
0.25-
I I I I I I I
470 550 670 700 720 750 790
Wavelength (nm)
Figure 6. Comparison of the WSN and ASD data at 4 different
locations. Sensor locations are represented here as
node numbers, which were assigned during the mote
board programming.
Comparison of data from WSN and ASD clearly indicated that
both sensors followed similar trends in monitoring crop
characteristics (Figure 7).
0.30-
Figure 5. Aerial image of the study site showing the sensor
locations.
Yellow dots indicate the 90 sample locations with base poles
inserted. These locations were determined based on
4 sampling strategies. The triangles indicate the
locations of the 14 functioning sensor units in 2009,
which followed a systematic sampling pattern.
Square symbols indicate the locations of the base
station to which all the motes sent data messages.
The aerial image shown here consists of three
narrow bands, 790, 720 and 670 nm which are
projected as red, green and blue, respectively.
o.oo-
470
700
720
790
Wavelength (nm)
Figure 7. Comparison of mean percentage of reflectance for
different wavelengths for the hyperspectral data
recorded using WSN and ASD.
Error bars indicate 95% confidence interval.
