validation of a high-resolution, remotely operated aerial remote-sensing system for the...
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Applied Vegetation Science 15 (2012) 383–389
Validation of a high-resolution, remotely operatedaerial remote-sensing system for the identification ofherbaceous plant species
Fumiko Ishihama, Yasuyuki Watabe & Hiroyuki Oguma
Keywords
High positioning accuracy; Non-destructive
survey; Portable remote-sensing system;
Radio-controlled helicopter; Wetland
Abbreviations
IMU = Inertial measurement unit; GCP =
Ground control point
Nomenclature
BG Plants Japanese-name-scientific-name Index
(YList), http://bean.bio.chiba-u.jp/bgplants/
ylist_main.html (accessed 30 November 2011)
Received 15 July 2010
Revised 30 November 2011
Accepted 20 December 2011
Co-ordinating Editor: Aaron Moody
Ishihama, F. (corresponding author,
[email protected]) &Oguma, H.
([email protected]): National Institute for
Environmental Studies, Onogawa, Tsukuba,
Ibaraki 305-8506, Japan
Watabe, Y. ([email protected]):
Information & Science Techno-System Co.,
Ltd., Takezono, Tsukuba, Ibaraki 305-0032,
Japan
Abstract
Question: Is a high-resolution remote-sensing system based on a radio-
controlled helicopter (the ‘Falcon-PARS system’) an effective tool to obtain
images that can be used to identify herbaceous species?
Location:Watarase wetland, Japan.
Methods: We applied the remote-sensing system to a wetland composed
mainly of Phragmites australis andMiscanthus sacchariflorus. The aerial observation
was performed in a 100 9 200 m area at a flying height of 30 m. From the
obtained images, we tried to identify P. australis and M. sacchariflorus through
visual interpretation.
Results: We obtained images with a high spatial resolution (1 cm) and a posi-
tioning accuracy of finer than 1 m using this small and lightweight system, and
confirmed that we could identify the above two species from the obtained
images.
Conclusion: Such a high-resolution system can be used to directly identify her-
baceous species, and as a non-destructive alternative to ground surveys. This
lightweight system can be carried to sites such as a high-altitude bog that cannot
be reached by a motor vehicle. Because of the low flying height (below cloud
level), aerial observation is possible even on cloudy days, thereby permitting
observations in all seasons.
Introduction
Remote sensing is a convenient tool for efficient, non-
destructive mapping of vegetation over wide spatial scales.
Satellite and aircraft remote sensing is widely used to
obtain distribution maps of vegetation classification (De-
Fries 2008; Xie et al. 2008; Hill et al. 2010) and habitat
maps of species (Kerr & Ostrovsky 2003), and to estimate
biomass (e.g. Boudreau et al. 2008) and plant phenology
(Verbesselt et al. 2010; Reed et al. 2009). Although these
remote-sensing systems are effective for such observations,
they are only useful for relatively large targets, such as tall
trees, or for rough classification of vegetation types. This is
because the resolution of these systems is relatively low
(5 cm at best for aircraft remote sensing). To identify her-
baceous or small woody species or to classify vegetation
type in detail, fine-scale remote sensing with a resolution
of � 1 cmwould be required.
Although there is an inevitable trade-off between reso-
lution and observation speed, a high-resolution remote-
sensing system capable of distinguishing among detailed
vegetation types or identifying small plant species has
advantages that outweigh its reduced speed. The first is
that it permits non-destructive observation. Ground sur-
veys sometimes cause substantial damage to the vegeta-
tion, particularly at fragile sites such as bogs. Although
long-term monitoring is required to examine changes in
biodiversity and to plan effective conservation measures
(Marsh & Trenham 2008), damage to vegetation during
monitoring on foot can be especially serious when
repeated surveys are required. Remote sensing with suffi-
ciently high resolution would be a valuable alternative to
Applied Vegetation ScienceDoi: 10.1111/j.1654-109X.2012.01184.x© 2012 International Association for Vegetation Science 383
ground surveys because it would reduce or eliminate dam-
age to vegetation. A second advantage is the ability to
obtain detailed observations of sites that are difficult for
humans to approach, such as cliff faces and the canopies of
tall trees. Third, even if the speed is relatively limited,
high-resolution remote sensing still provides a faster tool
for mapping individual plants than is possible in surveys
conducted on foot.
The criteria for a remote-sensing system suitable for
high-resolution observation include high positioning accu-
racy, a robust ability to work under a range of weather
conditions, and portability (light weight). High positioning
accuracy is essential to allow comparison of images from
different times so that researchers can monitor temporal
changes in vegetation and can overlay images with other
geographical information, such as elevation. Robustness
under a range of weather conditions is required to permit
surveys in all seasons. Phenological changes represent
information that can be used to distinguish plant species,
and multi-seasonal observations capable of detecting phe-
nological changes are an effective way to distinguish plant
species or vegetation types (Gilmore et al. 2008). Remote
sensing from piloted aircraft is possible only under a lim-
ited range of weather conditions (i.e. clear days) because
the piloted aircraft fly as high as 2000 m, and their sensors
may be blocked by low cloud. Obtaining a cloud-free
image is also an important problem for satellite remote
sensing (Xie et al. 2008; Wang et al. 2009). Such limita-
tions often make it difficult to perform surveys in certain
seasons. Portable systems would be required at study sites
such as those at high altitudes, wetlands and oceanic
islands, which are usually inaccessible to ground vehicles.
Remote sensing using a radio-controlled helicopter,
fixed-wing aircraft and balloon is a potential candidate for
high-resolution remote sensing because such vehicles can
fly at much lower altitudes than piloted aircraft. The effec-
tiveness of these systems for ecological or agricultural sur-
veys that require resolutions ranging from several meters
to several tens of centimeters has been reported (Davis &
Johnson 1991; Gerard et al. 1997; Johnson et al. 2004;
Miyamoto et al. 2004; Sugiura et al. 2005; Berni et al.
2009; Artigas & Pechmann 2010). However, some of these
systems are not suitable to capture georeferenced high-
resolution images at resolutions of 1 cm or finer in a non-
destructive way. A balloon system is very vulnerable to
wind, and it is difficult to control its position, especially in
high-resolution surveys, which require delicate position-
ing control with accuracy finer than a fewmeters. Because
tethered balloon systems need to be towed by a human for
positioning control, they can cause damage to vegetation
in study sites susceptible to trampling. In addition, balloon
systems require containers of pressurized, lighter-than-air
gas, which cannot be carried by humans over long dis-
tances to reach remote sites. Although fixed-wing aircraft
have superior positioning control and robustness against
wind, their high flight speed can cause serious problems;
obtaining high-resolution images with sufficiently high
positioning accuracy faces many specific problems (e.g.
motion blur in the images due to a combination of insuffi-
cient light and an insufficiently high camera shutter
speed). These problems can be solved by flying more
slowly or by hovering, if the aircraft has a low level of
vibration (Appendix S1). In addition, a fixed-wing aircraft
often requires flight strips for takeoff and landing, and
these are rarely available in survey areas.
To solve these problems, we chose a lightweight
remote-sensing system capable of hovering and with
low vibration. To meet these criteria, we chose a heli-
copter (AscTec Falcon 8; Ascending Technologies GmbH,
Krailling, Germany; Fig. 1a) that can hover at the
assigned coordinates (using an autopilot function) and
obtain photographs by automatically activating the cam-
era shutter. It is only in the last few years that light-
weight radio-controlled remote-sensing systems with an
(a) (b)
Fig. 1. (a) The helicopter (AscTec Falcon 8; Ascending Technologies GmbH, Krailling, Germany) and (b) camera used in the high-resolution remote sensing
system.
Applied Vegetation Science384 Doi: 10.1111/j.1654-109X.2012.01184.x© 2012 International Association for Vegetation Science
High resolution remote-sensing system F. Ishihama et al.
autopilot function became available. The autopilot func-
tion allows the aircraft to fly along a predefined course
and obtain photographs automatically at preset coordi-
nates, and it is therefore an essential function for easy
and speedy image acquisition. Such systems have been
developed mainly for military (Newcome 2004) or geo-
graphical use (e.g. Delacourt et al. 2009), so their appli-
cability to plant surveys has rarely been evaluated (but
see Rango et al. 2009).
In this study, we aimed to validate the use of a remote-
sensing system based on a radio-controlled helicopter to
examine whether it could satisfy our criteria (high resolu-
tion, positioning accuracy, robustness across a range of
weather conditions, and portability) for monitoring of her-
baceous plants. We tested whether we could use images
obtained by this system to distinguish among herbaceous
plants species in theWatarase wetland, Japan.
Methods
The radio-controlled helicopter system
Thehelicopterusedinthisstudyissmall(85 9 80 9 15 cm)
andlight(1.6 kg, including itsbattery).Becausethehelicop-
ter has a small payload capacity (500 g), we used a
lightweight compact digital camera (GX200; Ricoh, Tokyo,
Japan; Fig. 1b) as the image sensor. The continuous flight
timeis<20 min.Thehorizontalflightrangeiswithin1 kmof
the operator due to radio control limitation, andmaximum
flight height is 300 m. The radio frequency of the control
system is 2.4 GHz. The helicopter includes an onboard
GPS(LEA;u-blox,Thalwil,Switzerland).
Although this small helicopter is suitable for high-
resolution photography, it is difficult to obtain high posi-
tional accuracy using only the onboard GPS. To obtain
highly accurate georeferencing capability and to allow us
to combinemultiple digital pictures into onemosaic image,
we used the Cartomaton software (Information & Science
Techno-System Co., Ltd., Tsukuba, Japan). Cartomaton
generates simple ortho-images (i.e. images corrected for
distortion caused by changes in flight attitude of an aircraft
and by chromatic and spherical aberration resulting from
the camera’s lens). This software estimates the external
orientation (three-dimensional position and angle) of the
camera when the photos are taken. After performing geo-
metric corrections based on those angles, the software pro-
jects the photographs onto a plane that is assumed to
represent the ground surface, and then combines all the
photographs into a single georeferenced mosaic image.
During this processing sequence, it uses side-by-side pairs
of photos to calculate an external orientation; thus, it does
not require an inertial measurement unit (IMU) or ground
control points (GCP) to achieve precise corrections of
distortion.
We have named this system (helicopter, digital camera
and Cartomaton software) the ‘Falcon- photogrammetry
and remote-sensing (PARS)’ system.
Study site for the aerial observation of vegetation
We tested the Falcon-PARS system in the Watarase wet-
land of central Japan (139°41′ E, 36°14′ N, 14 m a.s.l.;
Fig. 2a). TheWatarasewetland is a floodplain wetland that
covers about 1500 ha, and its vegetation is mainly com-
posed of Phragmites australis (Cav.) Trin. ex Steud. and
Miscanthus sacchariflorus (Maxim.) Benth. Because these
species form dense vegetation that reaches a maximum
height of 4 m in July, ground surveys are impractical, and
remote sensing is therefore an essential monitoring tool.
Although a previous study reported successful detection of
expansion of pure stands of P. australis using a balloon sys-
tem with 12-cm spatial resolution (Artigas & Pechmann
2010), the species forms extremely mixed stands with
M. sacchariflorus in the Watarase wetland, and finer spatial
resolution is required for distinguishing these two species
in this wetland.
Conditions during aerial observations of the vegetation
We performed the aerial observations on 10 July 2009. The
weather was cloudy. We set the digital camera’s focal
length at 24 mm, shutter speed at 1/500 s, diaphragm at
F5.1 and ISO setting at 200. The camerahas an effective res-
olution of 12.1 megapixels. Our preliminary survey
revealed that amaximumflyingheight of 30 mwasneeded
to distinguish between P. australis and M. sacchariflorus
(F. Ishihama et al., unpublished data) using these camera
settings, so we performed the survey at this height. Image
resolution is a function of the flying height, effective pixel
resolution and focal length. Given the above-mentioned
settings, the spatial resolution of our images was 1 cm and
the areal footprint of each imagewas 30 9 40 m.
We performed aerial observations in a 100 9 200-m
area. To cover this area, we took 99 photographs to allow
for overlap between adjacent images within a given flight
line and side-lap between adjacent flight lines. The Car-
tomaton software requires 60% overlap and 30% side-lap
to combine the photographs into a single mosaic image; in
our study, we used 75% overlap and 30% side-lap.
Study site for testing positional accuracy
To test the positional accuracy of the ortho-mosaic image
and digital surface map (DSM) generated by the Falcon-
PARS system, we conducted aerial observations in a
research field at the National Institute for Environmental
Studies (140°4′41″ E, 36°3′3″ N). We chose this field as a
Applied Vegetation ScienceDoi: 10.1111/j.1654-109X.2012.01184.x© 2012 International Association for Vegetation Science 385
F. Ishihama et al. High resolution remote-sensing system
study site because it was difficult to establish a sufficient
number of GCPs throughout the survey area in the
Watarase wetland due to the extremely dense and tall
vegetation; thisvegetationmadeitnearly impossible towalk
at the study site, which is why high-resolution remote-
sensingobservationsare required formonitoringof this site.
Conditions for testing positional accuracy
We performed the aerial observations on 23 February
2011. The camera settings, flying height and overlap and
side-lap settings were the same as those used in our obser-
vations of the wetland vegetation.
We established ten 10 9 10-cm plates as GCPs, and
used these GCPs to evaluate the position accuracy of
mosaic images. The coordinates of the ground control
points were obtained using a two-carrier-wave-frequency
GPS (Geodetic IV; Ashtech, Carquefou, France). The stan-
dard deviations of the positioning accuracies of all GCPs
obtained with the GPS were <1 cm. The flight covered a
70 9 50-m area and we obtained 20 photographs (five
photographs per course).
After the photography, we performed baseline analysis
using the raw data from the onboard GPS. By using these
photographs and the analysed GPS data, we generated a
true ortho-mosaic image and DSM; we did not use any
GCP data to create these mosaic images.
Test of repeatability of the classification of plant species
from the aerial image
To test the repeatability of species classification based on
the mosaic image obtained from the aerial observation in
Watarase wetland, we performed classification of plant
(a) (b)
(c) (d)
Fig. 2. (a) Location of the study site, the Watarase wetland, in Japan. (b) A simple ortho-image obtained by the radio-controlled helicopter remote-sensing
system. (c) A sample magnified image [location marked by light blue box in (b)]. (d) The resulting map of the species distribution identified from visual
interpretation of the magnified images.
Applied Vegetation Science386 Doi: 10.1111/j.1654-109X.2012.01184.x© 2012 International Association for Vegetation Science
High resolution remote-sensing system F. Ishihama et al.
species by means of visual interpretation using three photo
interpreters. The three photo interpreters had different
experience in vegetation research: one was an experienced
plant ecologist, the second was a remote-sensing
researcher with little experience in vegetation surveys,
and the third was a non-researcher who had experience
assisting in vegetation surveys. Before the test, we taught
the photo interpreters the criteria they should use to distin-
guish among the three categories. Appendix S2 shows the
tutorial materials that were used.
As samples of a classification test, we first selected 200
random 30 9 30-cm test image areas within the image.
Then we omitted test image areas that meet at least one of
the following four criteria: (1) the image did not include
either P. australis or M. sacchariflorus; (2) the image
included both P. australis and M. sacchariflorus; (3) the
image was too dark because the area is composed of low
plants shaded by surrounding tall plants; and (4) the image
was blurred due to movement of leaves by wind. We omit-
ted the image that matched criteria 1 and 2 because such
areas require classification categories such as ‘other plants’
and ‘both P. australis and M. sacchariflorus’ in the test. Set-
ting such categories can inflate repeatability of classifica-
tion, because it is expected that photo interpreters tend to
choose these categories when they are not sure.
Finally, we used 100 image areas for classification tests.
We asked each photo interpreter to classify the species of
the plant at the test areas using two categories: P. australis
andM. sacchariflorus.
Results
Aerial observations of vegetation in theWatarase
wetland
To obtain an image of the whole 100 9 200-m study area
from a flying height of 30 m, it took only 11 min and 10 s.
We obtained clear images with sufficient overlap and side-
lap, and were able to create a high-resolution mosaic
image from the simple ortho-images (Fig. 2 b,c).
Test of positional accuracy
We calculated the root-mean-square errors (RMSEs) for
the positions measured in the field at the National Institute
for Environmental Studies. The RMSEs were 0.974 and
0.360 m for the horizontal and ellipsoidal body height
positioning errors, respectively.
Repeatability of the classification of plant species from
the aerial images
The rate of agreement of the species classification among
the three photo interpreters was 84.0%, and the numbers
of image areas in which the three different interpreters
agreed or disagreed on species are shown in Table 1.When
we compared the classification by the two non-expert
photo interpreters to that of the experienced plant ecolo-
gist, the rates of correct answers were 90.0% and 93.0%,
respectively.
Discussion
Because we used a helicopter that can hover above a
desired position, we did not experience any of the prob-
lems described in the Appendix S1: we obtained clear
images with sufficient overlap to create a mosaic image.
From the high-resolution mosaic image generated from
the simple ortho-images (Fig. 2b,c) we could distinguish
both P. australis andM. sacchariflorus (also see Appendix S2
for ground images of these species) through visual inter-
pretation, with high repeatability among photo interpret-
ers of different experience in vegetation research. An
example of classification by the experienced photo inter-
preter is shown in Fig. 2d. Because the resolution was
much higher than could be obtained using conventional
aerial photographs (Table 2), the photo interpreters could
use both colour differences and differences in form of the
leaves and structure of the plant bodies as clues to assist in
the identification of the two species. Because the colour
depends on weather conditions (e.g. light intensity and
quality) and season (e.g. summer vs autumn leaves)
Table 1. Repeatability of the classification of plant species from aerial
images. The numbers of image areas in which the three different interpret-
ers agreed or disagreed on species are shown.
Pattern of classification by three interpreters Number of
image areas
Three interpreters classified as
Phragmites australis
39
Two interpreters classified as
P. australis, one asMiscanthus sacchariflorus
6
One interpreter classified as
P. australis, two asM. sacchariflorus
10
Three interpreters classified asM. sacchariflorus 45
Table 2. Comparison of the characteristics of a remote-sensing system
with a piloted aircraft and the radio-controlled helicopter system validated
in this study (the Falcon-PARS system).
Ordinary aerial
photographs
from a piloted aircraft
Falcon-PARS
system (30-m
flying height)
Highest resolution 5 cm 1 cm*
Area photographed in 1 h Several km2 ca. 0.06 km2
Minimumweather
conditions
Clear day Bright cloudy day
*Finer resolution is possible at a lower flying height, but this decreases the
area that can be photographed per hour.
Applied Vegetation ScienceDoi: 10.1111/j.1654-109X.2012.01184.x© 2012 International Association for Vegetation Science 387
F. Ishihama et al. High resolution remote-sensing system
during the aerial observations, the form of the plants is a
more reliable clue to identify species.
We confirmed the ability of our system to provide a res-
olution of ca. 1 cm while imaging natural herbaceous veg-
etation. Although many studies (e.g. Lelong et al. 2008;
Berni et al. 2009) have used unmanned aerial vehicles
(UAVs), few of these systems have attained a spatial reso-
lution finer than 5 cm. The only system we are aware of
that provides 1-cm resolution is a helicopter-based UAV
system used for observation of coastal areas (Delacourt
et al. 2009). The other system attained resolutions of ca.
5 cm and was used to observe rangeland (Rango et al.
2009). Previous UAV systems that attained a high spatial
resolution (ranging from 1 to 5 cm) were large (1.0–
1.8 m) and heavy (10–11 kg, excluding image sensors)
and were therefore difficult to transport without ground
vehicles. Although such systems have some merits (larger
battery capacity and pay-load than the Falcon-PARS sys-
tem), it would be difficult to take them tomany study sites,
such as alpine sites. Our system is only 1.6 kg including
the battery (1.8 kg including the camera) and can there-
fore be transported by a single person to almost all possible
study sites. Moreover, our system does not need any exter-
nal orientation to obtain georeferenced images. This char-
acteristic further reduces difficulties in field surveys; this
system does not require setting GCPs in tall and dense veg-
etationwhere it is difficult to walk or in fragile bogs, or car-
rying a heavy two-way GPS to sites that are difficult for
humans to approach with heavy baggage, such as alpine
sites. The main drawbacks of our system are small battery
capacity (ca. 20 min of continuous flight time) and small
payload (ca. 500 g), but its portability outweighs these
drawbacks for sites such as bogs that are difficult to reach
with a vehicle and too fragile to survey on foot. It should
also be noted that although the imagery has a spatial reso-
lution of 1 cm, which allows for fine-scale image interpre-
tation, the positional accuracy of ca. 1 m limits the
resolution of vegetation classification to larger areas in
which the positional error is negligible.
It took only 11 min and 10 s to obtain an image of the
entire 100 9 200-m study area from a flying height of
30 m. A survey performed at this speed covers a much
smaller area than would be covered by conventional aerial
remote sensing with a piloted aircraft (Table 2) because of
the inevitable trade-off between image resolution and fly-
ing speed. However, this was still remarkably faster than
would have been possible by means of a ground survey.
Even though additional time is required to process the data
to produce the true ortho-image (5–6 h per 100 photo-
graphs, although this time varies depending on perfor-
mance of a personal computer) and classify the vegetation
types from the image, we were nonetheless able to rapidly
record the status of vegetation. This is important for
researchers because some vegetation changes its state so
quickly that ground surveys cannot be performed suffi-
ciently rapidly to cover the whole study area before it
changes (e.g. the state of flushing of spring ephemerals
changes within a few days). In addition, this approach per-
mits non-destructive surveys, which is very useful at frag-
ile sites such as bogs. The approach also makes it possible
to monitor sites such as tree canopies or cliff faces that
would be difficult or impossible to study in any other way.
The system’s portability (small size and light weight) is an
additional advantage for use in such places.
The studied species at the study site were relatively large
grasses, making the plant characteristics easy to distin-
guish, but application to smaller herbs should be possible
using a lower flying height. Although observation speed
decreases at a lower height, better resolutionwill be attain-
able in the near future using a camera with a larger num-
ber of pixels without reducing flying height. We used a
common compact digital camera with a relatively small
number of effective pixels, but the remarkable speed of
development of digital cameras suggests that higher image
quality will soon provide the same image resolution from a
greater flying height (i.e. will allow observation of a larger
area per unit time). In addition to high-resolution cameras,
researchers can also use other sensors such as near-
infrared cameras, which can be used tomeasure plant pho-
tosynthetic activity. The only limitation of the system is
that the sensor must be light enough to mount on to the
radio-controlled helicopter.
The Falcon-PARS system is a promising tool for efficient,
non-destructive surveys of herbaceous vegetation.
Although we identified plant species by eye in the present
study, the development of image analysis techniques to
automatically identify species will further improve the
applicability of this system in the near future.
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Supporting Information
Additional supporting information may be found in the
online version of this article:
Appendix S1. Possible problems in high-resolution
remote sensing (with resolution finer than 1 cm) and
solutions.
Appendix S2. Ground-level photographs of the
leaves and plant bodies of (a,c) Phragmites australis and
(b,d) Miscanthus sacchariflorus. The remote-sensing
images magnified from Fig. 1(c,d): (e) P. australis and (f)
M. sacchariflorus.
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Applied Vegetation ScienceDoi: 10.1111/j.1654-109X.2012.01184.x© 2012 International Association for Vegetation Science 389
F. Ishihama et al. High resolution remote-sensing system
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