1. The possibility to detect single oil palm trees.
2. The possibility to detect small oil palm trees that are
in an early growth stage. 3.
The possibility to derive tree density information on a per-hectare basis for an individual block and for the
complete plantation. 4.
The possibility to derive information about size and health of each individual tree.
5. The possibility to manually improve quality of the
automatically generated result by smart and efficient tools. 6.
The possibility to export analysis results into a GIS environment.
The complexity of the analysis and the requirements on usability lead to the conclusion that an object-based image
analysis OBIA approach is suitable to meet the analysis requirements. Trimble’s eCognition software has OBIA
capabilities and facilitates an environment to develop standalone applications meeting technical requirements as well
as matching requirements for usability of this type of project.
3. RESULTS AND DISCUSSION
3.1 Spatial Accuracy with or without GCPs
3.1.1 Reliability analysis: A total of 25 test flights have
been conducted using all lens types and a range of GSD values. Details of the test flights and a comprehensive report are
described by Pauly Pauly 2016. Results and key learnings are discussed below s. table in Appedix.
Given optimal flight planning 80 planned overlap and flight lines perpendicular to the wind, no relation between accuracy
and weather conditions was discovered. In case where manual quality assurance steps were conducted to assure absolute
accuracy, the applied shift correction was in Z only and was 14.9 cm on average, with a minimum of 3.7 cm and a maximum
of 51.8 cm. Overall, an average root-mean-square error RMSE of 2.7
pixels horizontal and 2.4 pixels vertical was obtained. Figure 4 shows these values as a function of the GSD,
expressed in meters and times the GSD, respectively. As expected, there is a strong linear relation with GSD for both
horizontal and vertical accuracy when expressed in meters figure 4.
Figure 4: Horizontal and vertical RMSE in meters vertical axis for different GSDs in cm horizontal axis. H = horizontal, V =
vertical. Ultimately, for a smaller GSD, it is the accuracy of the GNSS
control and check point measurements in the field that will determine the ASPRS accuracy class of the results, because
those will supersede the accuracies of the photogrammetric processing ASPRS 2015, Whitehead Hugenholtz 2015.
3.1.2
Accuracy relative to the number of GCPs: Two
selected flights have been further analysed. GCPs have been measured by RTK using a Trimble R10 GNSS survey
instrument in a VRS network resulting in an average horizontal accuracy of 1.3 cm and vertical accuracy of 1.7 cm.
Table 1 and 2 show the accuracies for the two flights: Flight 020
With trajectory, no GCPs 18 CP
No trajectory, 9 GCPs 9CP
RMSE
x
on CP 0.033
0.026 RMSE
y
on CP 0.025
0.013 RMSE
z
on CP 0.034
0.058 Table 1: RMSEs in m for flight 020, 2.4 cm GSD.
Flight 015 No GCPs 17 CP, 3
additional lines No GCPs 17 CP,
main block 1 GCP 16 CP,
main block 9 GCPs 6 CP,
main block RMSE
x
on CP 0.025
0.052 0.055
0.062 RMSE
y
on CP 0.034
0.063 0.047
0.024 RMSE
z
on CP 0.032
0.075 0.026
0.064 Table 2: RMSEs in m for flight 015, 1 cm GSD.
This contribution has been peer-reviewed. doi:10.5194isprsarchives-XLI-B1-1113-2016
1116
The horizontal RMSE of the adjustment of flight 020 would meet the accuracy requirements for the 5 cm RMSE class
defined by ASPRS 2015. This means that for the final map product, the expected horizontal accuracy class would be 10 cm
according to ASPRS. However, it was discovered that the horizontal RMSE on final map products reflects the results from
the adjustment much more closely than the stipulated factor 2, since horizontal noise in the production of true orthomosaics is
much less of an issue than vertical noise in the DSM. While the processing strategy without trajectory, but with nine
GCPs equivalent to processing UX5 data resulted in a better horizontal accuracy, the strategy without GCPs using a post-
processed trajectory outperformed the conventional result for absolute vertical accuracy. Regardless, the RMSE values
resulting from both strategies would place the outcomes in the same ASPRS accuracy class both horizontally and vertically,
meaning in practice that the results from both strategies are equally accurate. This shows the validity of surveying oil palm
plantations with a minimal use of ground control points using the Trimble UX5 HP; an advantage considering that good GCP
surveys are often difficult or impossible inside large and mature oil palm plantations.
Figure 5 illustrates the increase in detail that can be seen in 1 cm GSD deliverables as compared to 2.4 cm GSD deliverables.
Due to the smaller features visible in the 1 cm imagery, points can be measured more precisely and accurately compared to the
2.4 cm imagery.
Figure 5: Demonstration of different GSDs top: 2.4 cm, bottom: 1 cm.
Even though the vertical accuracy of Flight015 relative to its GSD 2.6 pixels in the best strategy is worse than that of
Flight020 1.4 pixels, induced by the lower base:height-ratio and the less oblique viewing angles in Flight015, the absolute
accuracy is still slightly better for Flight015 owing to the small GSD, especially in the strategies where steps were taken to
eliminate any global shifts. The horizontal accuracies achieved from processing flights 020
and 015 without any ground control points are very comparable and fall under the same ASPRS accuracy class, and are likely
determined more by the on-board GNSS accuracy than the GSD. This shows that repeated UAS surveys can be done
without GCPs while still ensuring that deliverables align over time with a 1-2 pixel level accuracy, which is very important for
multitemporal analyses. 3.2
Automated software-based Oil Palm Monitoring
Whilst the described hardware and processing software has proven to generate results that are accurate enough for a
detailed analysis, the efforts to develop an automated analysis application were focused on two main components:
1. Stable, meaningful transferable analysis
capabilities. 2.
User Interface design to reflect requirements for usability in the given environment.
3.2.1 Oil palm tree detection using template matching:
