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Digital Aerial Imagery of Unmanned Aerial Vehicle
for Various Applications
Anuar Ahmad, Khairul Nizam Tahar, Wani Sofia Udin, Khairil Afendy Hashim, NorHadija Darwin, Mohd Hafis
Mohd Room, Nurul Farhah Adul Hamid, Noor Aniqah Mohd Azhar & Shahrul Mardhiah Azmi
Department of Geoinformation
Faculty of Geoinformation & Real Estate,
Universiti Teknologi Malaysia,
81310 UTM Johor Bahru, Johor, MALAYSIA
anuarahmad@utm.my
Abstract—Digital aerial imagery (DAI) can be acquired using
digital mapping camera attached to light aircraft. The DAI is
used for the production of topographic and thematic map. The
cost of acquiring DAI is very expensive and suitable for large
area coverage. The acquisition of DAI is not economical and
suitable for small area coverage. Therefore an alternative method
should be used to fulfill this need. There are two alternative
methods that can be used for acquisition of DAI which include
using a small format digital camera attached to light aircraft and
using a small format attached to an unmanned aerial vehicle
(UAV). UAV system has been reported used in various and
diversified applications such as mapping applications (eg. map
revision, landslide, coastal erosion, archaeology, forestry),
industrial application (eg. engineering, crash accident),
Geographic Information System (GIS) applications and others.
In this study, micro unmanned aerial vehicle (UAV) systems
which comprise of fixed wing UAV flying and rotary UAV are
attached with small format high resolution digital camera to
acquire DAI for the purpose of mapping at the flying height of
300m at 100m respectively. The micro UAVs were flown
autonomously (i.e automatically) and a series of DAIs of a slope
using fixed wing UAV and a stream using rotary UAV were
acquired rapidly within short period. Ground control point
(GCP) and check point (CP) were established using the Global
Positioning System and conventional Total Station techniques
around the study area for the slope and stream respectively for
the purpose of digital image processing and accuracy assessment.
The DAIs were processed to produce photogrammetric output
such as digital elevation model (DEM) and orthophoto. All these
photogrammetric products were successfully produced and
assessed. The achievable accuracy is less than ±1m for slope
mapping and ±0.280m for stream mapping. In this study, it is
proven that the micro UAV system can be used for mapping
which cover small area. As conclusion, micro UAV is suitable for
mapping small area, rapid data acquisition, accurate, low cost
and can be employed for various applications.
Keywords – Unmanned aerial vehicle, digital camera, digital
aerial imagery, photogrammetry, mapping
I. INTRODUCTION
Currently, many mapping organisations around the
world still used large format aerial camera for acquisition of
large format aerial photograph for the production of
topographic map. However, some mapping organisations have
started using digital mapping camera of different make for
mapping. However, due the high cost of the digital mapping
camera only not many mapping organisation afford to use it
even though it can deliver accurate and rapid photogrammetric
output. Usually, the procedure in conventional aerial
photogrammetry is lengthy and very costly to produce
topographic map because the whole process of mapping
involves many stages which include producing flight map,
acquisition of aerial photograph, establishment of ground
control point and lengthy image processing procedure. Also
this procedure is suitable for mapping large area only.
However, there are instances where aerial photograph which
covers small area is required for mapping purposes. The large
format aerial camera is not economical for mapping small
area. To overcome this problem, photogrammetrist has started
using small format camera for acquiring aerial photograph.
Research on the use of small format camera for mapping has
been conducted and promising results have been achieved [1],
[2],[3],[4],[5]. Photogrammetric output such as digital map
and orthophoto can be successfully obtained from small
format camera. Further research has been conducted on the use
of light platform for acquiring aerial photograph for the
purpose of mapping and other purposes. Today, it is common
for people from all around the world using small format
camera such as high resolution digital camera, video camera
and other sensor combined with light platform such as
helicopters, gliders, balloon and etc for acquiring digital aerial
images/photographs. For the light platform, it can be remotely
control manually or it can fly autonomously (i.e automatically)
based on pre-programmed flight plans or more complex
dynamic automation systems. The combination of the sensor,
light platform and procedure of flying the system without pilot
is known as unmanned aerial vehicle (UAV). In Malaysia, the
use of UAV for mapping and other applications is still new.
UAV has been used successfully for large scale mapping for
the production of photogrammetric output such as digital map,
digital orthophoto, digital elevation model (DEM) and contour
line [6],[7],[8],[9],[10],[11],[12],[13],[14].
There are a wide variety UAV shapes, sizes, configurations,
and characteristics. The earliest UAV, the Hewitt-Sperry
Automatic Airplane was developed during and after World
War I. During the World War II, a number of remote-
controlled airplane advances were made in the technology
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rush. These were used to train anti-aircraft gunners and to fly
attack missions. With the maturing and miniaturization of
applicable technologies as seen in the 1980s and 1990s,
interest in UAVs grew within the military. UAVs were seen to
offer the possibility of cheaper, more capable fighting
machines that can be used without risk to aircrews. Initial
generations were primarily surveillance aircraft, but some
were fitted with weaponry (such as the MQ-1 Predator, which
utilized AGM-114 Hellfire air-to-ground missiles).
II. MAPPING BASED ON AERIAL
PHOTOGRAMMETRY TECHNIQUE
In aerial photogrammetry, the flight planning must be
carefully planned and executed to secure good result. The
most important task of aerial photogrammetry is to constitute
an aid in production of topographic maps. A primary requisite
for rational utilization of photogrammetry is that the type and
the tolerances of the map desired are clearly determined at an
early stage of the working procedure.
In aerial photogrammetry it is a common practice to used
metric camera for acquiring aerial photograph and later
processed to produce topographic map. Aerial
photogrammetry using manned aircraft has been used for
many years and it is very efficient for large area. Manned
aerial photogrammetry is also used to update new areas which
need to be included in the existing topographic map.
Previously, manned aerial photogrammetry uses film as the
raw images of the earth surface but now it has been converted
into digital images. Today, digital mapping camera such
DMC, ADS 40, Vexcel etc which produce direct digital aerial
images (DAI) has been widely used by mapping organization
who afford to purchase it since digital aerial images could be
acquired rapidly and photogrammetric output could be
produced rapidly too. In photogrammetry, close range
photogrammetry (CRP) is a branch of photogrammetry which
can be utilized for mapping including aerial mapping. CRP is
suitable for small area or focused at the specific object to
fulfill the project needs. CRP is used in many applications
such as cultural heritage recording and in architectural
surveying. The conventional way in both photogrammetry
methods (i.e aerial photogrammetry and CRP) permit the 3D
model of the terrain to be produced and, by means of digital
elevation model (DEM), to realize multi-temporal studies. The
massive introduction of modern digital photogrammetric
workstations, with automatic matching procedures, allows for
a rapid DEM production.
Small format camera (i.e metric camera) and non-metric
camera such as digital camera, video camera etc could also be
employed to acquire DAI. The DAIs produced from non-
metric camera can be used for various applications such as for
map revision in GIS, research work/project and any
applications which do not require high accuracy. The non-
metric camera especially the digital camera offers several
advantages compared to metric camera. Some examples of the
advantages are ease of use, handy, cheap and the images are in
digital form which is ready to be used.
A. Digital photogrammetry and UAV
At present there are many digital photogrammetric systems
available in the market. In general, these digital
photogrammetric systems can process satellite imageries and
aerial photographs of metric or non-metric imageries. Today,
non-metric camera with high resolution could be used to
acquire aerial imagery. A digital camera of high resolution has
been used for the acquisition of aerial photograph [3],[4],[5].
In other study, an unmanned aerial vehicle (UAV) has been
used for the acquisition of high resolution DAI [15]. Also
UAV was used to acquire DAI and successfully produced
orthophoto [6],[7],[8],[9],[10],[11],[12],[13],[14],[16], [19].
In the last few years, UAV has received an increasing
interest as one of the reliable methods for slope studies. Since
the reality is three-dimensional (3D), it is a great advantage to
conduct modeling in 3D environment. Today, UAV has made
it possible to efficiently process and visualize data in 3D. It is
especially important to acquire fast and accurate 3D geometric
and visual information with minimum costs [6]. UAV is one
of the surveying methods conceived years ago that in a short
time it can supply digital elevation model (DEM) and good
quality digital terrain model (DTM) as a result of elaborations
with specific procedures. The main advantage of UAV over
traditional surveying techniques is its property of direct, rapid
and detailed image capture of study area. According to [17],
the advantages of UAVs are; low in cost, flexible, high
resolution images, able to fly under cloud, easy to launch and
land, and very safe. Other advantages are the dramatic
reduction in costs and much faster project completion,
possibility to survey remotely very complex, inaccessible and
hazardous objects and areas, where the conventional
techniques failed. The disadvantages of UAV include payload
limitation, small coverage for one image, increasing numbers
of images that need to be processed, and large geometric
distortion.
UAV is capable to fly in an autonomous way and operates
in a wide range of missions and emergencies that can be
controlled from a ground base station. UAV consists of the
airframe, flight computer, payload, the mission/payload
controller, the base station and the communication
infrastructure. For UAV that has mass less than 5kg, it is
known as micro UAV. Figure 1 shows example of micro fixed
wing UAV and rotary UAV system.
Fig. 1. (a) Fixed wing UAV (left) & (b) Rotary UAV (right)
The micro UAV airframe is a simple, lightweight,
aerodynamically efficient and stable platform with limited
space for avionics. The flight computer is a computer system
designed to collect aerodynamic information through a set of
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sensors (accelerometers, gyros, magnetometers, pressure
sensors, GPS, etc.), in order to automatically direct the flight
of an airplane along its flight-plan via several control surfaces
present in the airframe. The payload consists of sensors
composed of cameras, infrared sensors and thermal sensors to
gather information that can be partially processed on-board or
transmitted to a base for further analysis.
B. Cropcam UAV
Cropcam UAV is a product from Canada [18]. It is a new,
self-guided plane that creates GPS-based digital images. It was
originally designed to monitor agriculture crops (Figure 1(a)).
Using the Cropcam UAV, the user can scout disease, view
crop development and stop problems before they get out of
control. Cropcam UAV is inexpensive and easy to use with
preset flight plans. The DAI are accessible within hours and
provide latitude, longitude and altitude coordinates. The
Cropcam UAV is a radio controlled model glider plane
equipped with a Trimble GPS, a miniature autopilot and a high
resolution digital camera. It can be hand launched and
automatic from take off to landing. It is easily operated by
simply stand at one corner of an agriculture field and hand
launch the 2.7 kg UAV. The powerful miniature autopilot and
Trimble GPS, does the rest navigating in a pattern over the
field. Both the CropCam UAV and the Pentax Optio digital
camera with 12.0 megapixels perform automatically to take
GPS based DAI. After flight mission, the Cropcam UAV
landed at the spot it started automatically or the autonomous
flight can be override before landing to avoid damage to the
UAV.
The digital aerial photograph acquired by the Cropcam
UAV at 600m above the ground has a spatial resolution of
approximately 15cm. However, increased spatial resolution can
be achieved by simply programming the Cropcam UAV to fly
closer to the ground. In this study, the Cropcam UAV was
flown autonomously at 300m above the ground for the slope
mapping study area and the spatial resolution is approximately
8 cm.
C. Hexakopter UAV
Hexakopter UAV has 6 blades where 3 blades rotate
clockwise direction and 3 blades rotate counter-clockwise. A
high resolution digital camera is attached at the bottom of
Hexakopter UAV. The Hexakopter UAV is assembled with
complete set gadget such as GPS on board, pressure board,
speed board, gyro and mainboard (Figure 1(b)). The total
weight of the Hexakopter (i.e including the digital camera is
less than 5kg. The Hexakopter UAV can be flown
autonomously or manually. In this study, the Hexakopter UAV
was flown autonomously at 40-100m over a stream in the
study area and after reaching the required flying altitude, it
moved to the exposure station or way point for acquiring DAI.
After completing the flight mission for the first flying altitude,
the next flying altitude is performed according to the planned
flying altitude.
The Sony Alpha NEX-5N digital camera with interactive
16.1 megapixel and 3.0" touch liquid crystal display (LCD)
screen is attached to the Hexakopter UAV. At the flying
altitude of 300m, the ground spatial resolution is 5cm. The
digital camera needs to be calibrated for obtaining good
accuracy and results. It is calibrated with the purpose to obtain
the correct focal length and other camera calibration
parameters [11],[12]. Similarly, the digital camera attached to
the Cropcam UAV was calibrated too. These parameters were
substituted in the digital photogrammetric software together
with the GCPs for the process of aerial triangulation (AT). The
AT was successfully performed for digital aerial photographs
for both types of UAVs. The results of the digital image
processing and photogrammetric output are shown in the
following section.
III.METHODOLOGY
3D spatial information can be extracted from the digital
aerial photograph after the formation of the stereomodel that
can be viewed in 3D using special glasses. The Cropcam UAV
has been successfully employed in acquiring digital aerial
photograph and produced orthophoto [3],[4],[5]. The aim of
this study is to produce mapping product based on high
resolution DAI acquired from the micro UAV mentioned in
Section II. The flow chart of the research methodology is
shown in Figure 2 for the Crompcam UAV and Hexakopter
UAV for data acquisition and data processing.
Fig. 2. The flow chart of the research methodology
A. Slope Mapping
For slope mapping, the Cropcam UAV was used for
acquiring DAI of slope in the study area within Universiti
Teknologi Malaysia precinct. The UAV was flown at 300m.
The acquired raw DAI were processed by using digital
photogrammetric software. All the acquired images were
processed which involved interior orientation, exterior
orientation, aerial triangulation and bundle adjustment. Pixel
size is one of the important inputs in the interior orientation.
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This is because pixel size can determine the ground coverage
area of an image on the ground. Exterior orientation involves
the establishment of tie points and ground control points
(GCPs) between images. Tie points can be generated manually
or automatically. Manual editing requires a user’s
concentration in order to locate the point between two images
or in one model. User can also use automatic tie points
generation to establish tie point in the models. Automatic tie
points generation uses image matching correlation algorithm
to identify the same features in two images. However, user
needs to select good tie points and remove bad points after
running the automatic tie points operation. This step is
required to control the accuracy of the final results. GCPs
were established by using real time kinematic global
positioning system (RTK-GPS). Aerial triangulation is
performed after interior orientation and the accuracy of aerial
triangulation is analyzed using root mean square error
equation. There are two main products produced after the
photogrammetric process i.e digital elevation model (DEM)
and digital orthophoto.
B. Stream Mapping
The study area is an area within Universiti Teknologi
Malaysia, Johor, Malaysia. The first stage in the research
activities involves flight planning, photographic scale, flying
height of UAV, coverage and others are determined before
acquisition of DAI. It involved the determination of 60% side
lap and 30% end lap. A well-organized image requires an
essential arrangement because it is vital for data processing
and analysis. For stream mapping, the Hexakopter UAV was
used for data acquisition and these data are processed using
digital photogrammetric software. The GCP and CP were
established before the aerial photography mission. About 33
white crosses were painted as GCP which enclosed the study
area. The GCPs were fixed along both side of the stream flood
plain and coordinated by Total Station. Twenty-three (23)
points were used as GCP with full 3D (XYZ) coordinates
points and ten (10) points were used as CPs.
The DAIs were collected using the digital camera with
wide angle lens mounted on the Hexakopter UAV. The aerial
photographs were acquired in a straight line and form a series
of DAI. Flying height and speed were fixed, with variation in
flying altitude of 40m, 60m, 80m and 100m. A timing interval
was determined in order to obtain consistent flying height with
60% overlapping. One strip of the images in JPEG (Joint
Photographic Experts Group) for four different flying altitudes
was captured. Digital photogrammetric software was used to
perform data processing, generating digital elevation model
(DEM) and producing orthophoto of the stream. The GCPs
were used to perform the aerial triangulation in order to
produce 3D stereoscopic model. GCPs were also used to geo-
reference images to the local coordinate system. The step is
continued by generating DTM and orthophoto of the DAI.
IV. RESULTS
The results can be divided into two parts. The first part
shows the results of flying the fixed wing Cropcam UAV for
slope mapping and the second part shows the result of flying
the rotary wing Hexakopter UAV for stream mapping.
A. Slope Mapping
For this test, the two types of UAVs were used for
data acquisition and processed. In this paper, the results from
the fixed-wing Cropcam UAV are presented. Two primary
results were produced in this study namely DEM and digital
orthophoto. The slope map of the study area was also
produced. DEM and digital orthophoto were generated after
they went through all photogrammetric process. DEM is
generally based on the elevation value while digital orthophoto
consists of planimetric position x and y coordinates. The final
digital orthophoto can be obtained after mosaic operation by
using individual othoimages for each model in the
photogrammetric block. An accurate assessment of DEM and
digital orthophoto were carried out to determine the level of
photogrammetric results compared with ground truth
measurements.
The study area has the dimension of about 400 meter by
200 meter. However, the focus area of interest for accuracy
assessment is along the road which has various types of cut
slope and it is very suitable for slope error distribution analysis
in order to fulfill the objective of this study. The accurate
assessment of both results was completed by using root mean
square error equation. About 20 checkpoints were being
established evenly for the whole study area Figure 3 shows the
orthophoto, DEM and slope map of the study area. All ground
control points (GCP) and checkpoints (CP) were established
evenly to cover the area of interest including flat area, semi-
slope area and slope area. All checkpoints were also
established by using RTK-GPS.
In this study, accuracy assessment was performed based on
RMSE value. As mentioned in Section III, the GCPs and CPs
were established by using real time kinematic global
positioning system (RTK-GPS). The GCPs are used for the
production of DEM and digital orthophoto while the CPs are
used for accuracy assessment. Three types of RTK GPS
techniques have been conducted to fulfill the objective of this
study. For the first technique, RTK GPS received adjustment
from Iskandar Network for real time adjustment during the
control point observation. For the second technique, RTK GPS
received adjustment from master station which was set up at
the known GPS point and each point was observed for 2 min.
Finally, for the third technique, RTK GPS received adjustment
from master station has same result with the second technique
except the observation time is 10 min. Figure 4 shows the
comparison on photogrammetric results of slope mapping
based on different RTK GPS data.
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(a) Digital orthophoto (b) DEM
(c) Slope map
Fig. 3. (a)Digital orthophoto; (b) DEM; (c)Slope map
Fig. 4. Comparison of slope mapping results with RTK GPS data
Based on Fig. 4, it was found that the accuracy between the
first technique and the second techniques are almost similar.
Specifically, the different of x coordinate for the first and
second technique is about ±18 cm, the different of y
coordinate for the first and second technique is about ±19 cm
and the different of z coordinate for the first and second
technique is about ±22 cm. The range error for x, y and z
coordinates are ±1.300 to ±3.1 m. However, the third
technique gives different results from the first and second
technique. All errors of the third technique were recorded
below ±1m. Residual mean square error (RMSE) for x
coordinate recorded ±31 cm, y coordinate recorded ±34 cm
and z coordinate recorded ±77 cm. The difference between the
first and the second technique is huge compared to the third
technique. Based on the RMSE graph, it can be concluded that
the third technique is the best RTK GPS data for Cropcam
UAV image processing.
B. Stream Mapping
For this application, the rotary wing Hexakopter UAV was
used for data acquisition. After acquisition of the DAI of the
stream, digital image processing was performed. In this study,
a series of DAI from each flying altitude/height were used to
produce DEM and orthophoto. An orthophoto is a product
that has pictorial quality of a photograph and correct
planimetric characteristics. Orthophoto is produced through
the process of differential rectification whereby photo tilt, lens
distortion, and relief displacement have been eliminated and
adjusted. The orthophoto was created after the process of
aerial triangulation. Individual orthophoto was generated for
each individual DAI. The individual orthophoto was then
mosaic together to create a composite orthophoto. Digital
orthophoto only provides a two-dimensional view which
generally involves X and Y coordinates. Figure 5 depicts the
DEM of the stream and the orthophotos are shown in Figure 6
based on different flying altitude.
In this study, accuracy assessment was performed based on
RMSE value. Table 1 shows that the results of accuracy
assessment of digital orthophoto based on RMSE. Based on
the Table 1, it is clearly seen that the values of mean RMSE of
planimetric accuracy for all the flying altitudes are almost the
same with very slight difference. This indicates that the
planimetric accuracy can be considered uniform.
DEM of stream (40m ) DEM of stream (60m)
DEM of stream (80m ) DEM of stream (100m)
Fig. 5. DEM of stream at different flying height
Orthophoto of stream (40m ) Orthophoto of stream (60m)
Orthophoto of stream (80m ) Orthophoto of stream (100m)
Fig. 6. Orthophoto of the stream
2013 IEEE International Conference on Control System, Computing and Engineering, 29 Nov. - 1 Dec. 2013, Penang, Malaysia
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TABLE I. RMSE OF ORTHOPHOTO BASED ON VARIATION
FLYING ALTITUDE
RMSE (m)
Flying
altitude(m)
Aerial
Triangulation
X(m)
Y(m)
Mean(m)
40
10 CPs
±0.411
±0.156
±0.284
60
10 CPs
±0.415
±0.159
±0.287
80
10 CPs
±0.410
±0.163
±0.287
100
10 CPs
±0.415
±0.149
±0.282
For height accuracy, it is anticipated that there will be
significant since in photogrammetry the height accuracy is
normally double the planimetric accuracy.The slight
differences in planimetric accuracy might be affected by
image matching algorithm used in the image processing
software. The differences are usually caused by error in image
acquisition process such as motion movement such as omega,
phi and kappa and crabbing and image matching during image
processing.
VI. CONCLUSION
In this studym, it was found that the for slope mapping
RTK GPS from known point with 10 min observation gave the
best result in term of accuracy and precision. Based on this
observation, an accuracy of less than ±1m was achieved using
the Cropcam UAV flying at 300m. UAV is one of the efficient
equipment to obtain three dimensional model of the area of
interest especially at the slope area. For stream mapping based
on Hexakopter UAV flying at 100m, it was found that the
DEM and orthophoto were successfully produced and the
planimetric accuracy of the orthophoto is ±0.280 m.
This study proved that the UAV systems are capable of
acquiring DAI successfully and the mapping product can be
produced accurately within short period. The methodology
adopted in this study is useful and practical for large scale
mapping of small area and when budget is limited.
ACKNOWLEDGEMENT
The authors would like to acknowledge the support of
Faculty of Geoinformation & Real Estate, Universiti
Teknologi Malaysia (Grant No: Q.J130000.2527.03H68) and
Ministry of Education.
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