Capability of Low Cost Digital Camera for Production of Orthophoto and Volume

Determination Khairul Nizam Tahar*1 , Anuar Ahmad#2

*1Department of Surveying Science & Geomatics, Faculty of Architecture, Planning & Surveying Universiti Teknologi MARA

40450 Shah Alam, Selangor, Malaysia *1nizamtahar@gmail.com

#2Department of Geoinformatics, Faculty of Geoinformation Science & Engineering Universiti Teknologi Malaysia

81310 UTM Johor Bahru, Johor, Malaysia #2anuarahmad@utm.my

Abstract -This paper discusses about the use of low cost digital camera or known as non metric camera as photogrammetry tool for speeding up the process of acquiring aerial data. Digital camera can also be categorized as non metric camera. In photogrammetry, digital camera has been used for many diversified applications either in aerial photogrammetry or close range photogrammetry. The digital camera also has the potential to be used for landslide mapping and monitoring. In this study, a low cost digital camera has been used to acquire digital images of a simulated model build from sand and cement (dimension of 1m x 3m) which represent some form of topographic surface. In the simulated model, an area was excavated to simulate landslide occur in the area. The acquired digital images of the simulated model were then processed using photogrammetric technique to the final photogrammetric output that is the digital orthophoto. The digital camera used is of high resolution and suitable for large scale mapping which does not involve large study area, require moderate accuracy and limited budget. The objective of this study is to investigate the capability of a low cost digital camera in generating three dimensional models, digital elevation model (DEM) and finally the digital orthophoto. The other objective of this study is to investigate whether the data captured using the digital camera can be used for volume determination that was excavated from an area in the simulated model. The results of the study showed that the low cost digital camera is capable of producing digital orthophoto and capable of determining the excavated volume of the simulated model. Keywords- Low Cost Digital Camera; Orthophoto; Volume; DEM;

Photogrammetry

I. INTRODUCTION In general, digital technology has increased the quality of

data in mapping. Previously metric camera is attached to the

aircraft capturing aerial photograph or images from certain flying height. The development of information technology has impacted the accuracy in mapping task. Aerial mapping has increasingly become important for the last few years. There are many requests for large scale mapping which involve aerial photographs of small area. The famous source that is generally used by people is Google Earth. Google Earth utilizes satellite images and is commonly used as background data.

In 1858, the first person who introduced aerial mapping is Gaspar Felix Tournachon or known as ‘Nadar’. He photographs a French village at approximately 80 meters above earth surface from a balloon. In World War I and II the military produces aerial map for war strategy. Aerial mapping has been applied in many fields such as archeology, town planning, monitoring, natural hazard warning, environmental studies and surveillance. There are also many make and models of camera that can be used in capturing images of the earth surface. In photogrammetry, there are two types of camera widely known as metric camera and non metric camera. Metric camera has been used for a long time in photogrammetry mapping [6]. Metric camera are characterized by the technical features such as robust mechanical structure of the lens-camera system, high stability of the camera geometry: the interior orientation parameters remain unchanged and can be treated as known over a long period of time (fixed focal length, fixfocus), lenses are almost free of distortion, principal point offset is equal to zero, plane image surface by mechanical flattening and definition of an image coordinate system by fiducial marks. On the other hand, non metric camera is different from metric camera in many aspects [1]. Digital camera can be categorized as non metric camera since it was not designed for photogrammetric purpose. Digital camera is available in different resolution and pixel size. Pixel size is the camera picture element which needs

2011 IEEE 7th International Colloquium on Signal Processing and its Applications

67978-1-61284-413-8/11/$26.00 ©2011 IEEE

to be determined before photogrammetry work begins. Every digital camera has different image resolutions. The image resolution is defined by the sum of the number of horizontal pixel multiplied with the number of vertical pixel. In this study, Nikon Coolpix L4 digital camera has the image resolution of 2272 x 1704 pixels or about four Megapixels.

In photogrammetric method, a pair of images of 60% overlapped is commonly used and should comprise of well distribute ground control point in the overlapped area. Using the photogrammetric method, the digital elevation model (DEM) could be generated automatically with sufficient number of tie points established in the overlapped area. In this study, the digital images acquired from the digital camera images were processed to generate DEM automatically and subsequently, calculation of volume soil loss in landslide simulation is also carried out.

II. METHODOLOGY In this study, the methodology is divided into four phases

which include phase 1, phase 2, phase 3, and phase 4. Phase 1 is involves the preparation stage which include selection of study area, phase 2 is about data collection, phase 3 is about data processing of digital images and phase 4 is about result and analysis. All these phases are shown in Fig. 1.

Fig. 1 describes the methodology flowchart of this project. This study only concentrates on fixed platform to collect aerial photograph. The model was produced using sand and cement which only covered an area with dimension of three by one meter only. Before capturing images from an altitude, pixel size of the images should be considered in order to determine the size of one images using fixed focal length. The calculation of the size of one pixel of the digital image is as follows:

where; x= number of pixel of object image X= length of an object

F= focal length H= flying height

The calculation of the size of one pixel involves several

parameters such as scale, focal length of camera, flying height and images size in pixel. The ground coverage of the images from the digital camera could also be determined by multiplying the scale of the photography with the dimension of the digital image. The details of flight planning are shown in methodology flowchart where 11 photographs were captured to cover the whole model and involve one strip only and flying height of 1.2 meter. There are 33 ground control points distributed evenly for the whole model which were established using total station.

Fig. 1 Methodology Flowchart

III. DATA PROCESSING All the images were processed using Erdas Imagine

software. In the software, for digital camera the required information is only the camera focal length and sizes of one pixel for interior orientation. Exterior orientation involves ground control points to correct the digital images. Erdas

Phase 1

Phase 2

Establish Ground Control Point

Capture Digital Images

11 photograph

1 strip

1.2 meter

Developed Simulation model

Calculation of study area

Pixel size

Flying height

Digital Elevation

Orthophoto

Mosaic operation

Phase 4

Phase 3

Interior orientation

Exterior orientation

Aerial Triangulation

33 GCP & Tie Points

Analysis Calculation of Soil Loss

2011 IEEE 7th International Colloquium on Signal Processing and its Applications

68

Imagine software requires each pair of images should have at least three ground control points in order to perform aerial triangulation. Based on the photogrammetric concepts, every pair of photograph must overlapped at least 60 percent which include ground control points. The distribution of the ground control points and tie points for this project is shown in Fig. 2.

Fig. 2 Block for 11 photographs

Fig. 2 shows the footprint of 11 photographs that were captured and distribution of the ground control points and tie points after performing interior and exterior orientations.

IV. RESULTS After all images have been processed, there are two types

of output i.e; digital elevation model and orthophoto. In this study, 11 images have been mosaiced to cover the whole area. Fig. 3 shows the orthophoto of the study area and Fig. 4 shows the digital elevation model (DEM) of the simulated model.

Fig. 3 Orthophoto

Fig. 4 Digital Elevation Model Fig. 3 shows the orthophoto after interior and exterior orientations process have been performed. The orthophoto of each individual model were mosaics together in order to portray the whole simulated model from nadir angle without any distortion of images. Fig. 4 shows the DEM in raster form after performing aerial triangulation which is involved ground control points and tie points. Aerial triangulation uses concept of bundle adjustment for the process of determining the ground coordinates. Good quality ground control points are required in obtaining accurate DEM, hence, accurate ground control point

will produce an accurate exterior orientation, DEM and orthophoto.

V. ANALYSIS The aim of this study is to investigate the capability of the

camera in determination of orthophoto and volume determination. The assessment of landslide was processed using the DEM before and after landslide occurs. A portion of model has been simulating for landslide incident. The result of landslide simulation is shown in Fig. 5.

Before landslide

After landslide Fig. 5 DEM of landslide simulation

2011 IEEE 7th International Colloquium on Signal Processing and its Applications

69

Fig. 5 shows a location of landslide simulation of the simulated model. There are two situations in this figure; one image shows an area before landslide occurred and the other image shows the same area after landslide occurred. From these two different images, it can be viewed that there are differences in the DEM before landslide and after landslide occurred. The contour lines were generated for both situations for the purpose to determine the flow of landslide behavior. Note that the shape of the contour lines changed after landslide occurred (Fig. 6). Also this figure shows contour lines are superimposed with DEM at the landslide region.

Before After

Fig. 6 Contour line overlapping with DEM Fig. 6 describes a direction of contour lines before

landslide and after landslide happen. As a result, contour lines will follow a direction of landslide direction. From the DEM and orthophoto, these data also can be used to generate TIN (triangular irregular network) of the images. This study represent TIN model for visualizing the three dimensional model of the simulated landslide. Fig. 7 shows the three dimensional visualization before landslide and after landslide.

Before Landslide Landslide

After Landslide Fig. 7 Superimposition between TIN and contour lines before and after

landslide

The TIN models were produced using ArcGIS 9.3. This study also focuses on volume calculation using tool cut/fill in ArcGIS 9.3. Using this tool, volume and area of landslide area could be calculated automatically. The result of volume and area of landslide area is shown in Fig. 8.

Profile graph before landslide

Profilre graph after landslide

Fig. 8 Profile graph before and after landslide

Fig. 8 shows a profile graph of slope before and after landslide simulation. The minimum and maximum number of height value for each profile graph can be clearly seen. The calculation of volume is described in Fig. 9. This volume was calculated using cut/fill tools in ArcGIS.

2011 IEEE 7th International Colloquium on Signal Processing and its Applications

70

Fig. 9 Volume calculation

Fig. 9 shows the cut/fill result of the landslide area/region. This result was generated by using the DEM before landslide and after landslide. By overlapping these two images, the volume of landslide can be determined. Sum of soil loss of landslide is 0.002043 meter³ and the area of landslide is 0.000308 meter². These results were compared using conventional method. The real soil loss has been calculated in cylinder cube with diameter 23cm and height 5cm. So the volume calculation in cylinder cube is 2077.38cm³ or 0.002077 meter³. The difference of volume between these two methods is 0.000034 meter³ and can be considered as acceptable since the difference is too small.

VI. CONCLUSION & FUTURE WORK The orthophoto and DEM have been successfully produced

using the low cost digital camera. This study also suggests that the digital camera could be used for large scale mapping. The accuracy of result depends on good quality and accurate ground control point. Therefore it is very important to make sure the ground control is accurate. It also can be concluded that the digital camera is capable to produce three dimensional models and subsequently produces other photogrammetric output. This study also proves or demonstrates that landslide incident could be detected using digital camera with the condition that the area is small. This technology could be used by Public Work Department or Ministry of Environment to monitor area that is prone to landslide around Malaysia.

For future work, this research will focus on real location of landslide and real slope location. The study will also venture or explore the used of UAV (unmanned aerial vehicle) and high resolution digital camera for the process of acquiring aerial photograph for landslide prone area. The accuracy of DEM and orthophoto for the real area at landslide location will also be assessed.

VII. ACKNOWLEDGEMENT Faculty of Architecture, Planning and Surveying Universiti

Teknologi MARA (UiTM) and Faculty of Geoinformation & Real Estate, Universiti Teknologi Malaysia (UTM) are greatly acknowledged.

BIBLIOGRAPHY

[1] A. Ahmad. Study of Photogrammetric Mapping Accuracy using Low Altitude Unmanned Aerial Vehicle. International Symposium and Exhibition on Geoinformation, 2009.

[2] B. D. D. Rodríguez, S. H. G. García and Y. B. Piñero. Experience Using Non-Metric Cameras In Photogrammetry. The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B5. Beijing, 2008

[3] C. A. Rokhmana, Some Notes on Using Baloon Photography For Modeling the Landslide Area. Map Asia 2008.

[4] C. Y. Lo, L. C. Chen, L. Y. Chang and C. M. Huang. Hazard Survey Using Multitemporal Satellite Imagery. International Archives of the Photogrammetry, Remote Sensing and Spatial Information Science, Volume XXXVIII, Part 8, Kyoto, Japan, 2010.

[5] C.Y Lee, S.D Jones, C.J Bellman and L.Buxton. DEM Creation of snow covered surface using digital aerial photography. The international Archieves of the photogrammetry, Remote Sensing and Spatial Informatin Sciences. Vol XXXVII, PartB8. Beijing, China, 2008.

[6] J. Peipe and M. Stephani. Performance Evaluation Of A 5 Megapixel Digital Metric Camera For Use In Architectural Photogrammetry. The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol. XXXIV, Part 5/W12, 2005.

[7] P.R.Wolf & B. A. Dewitt, 2004. Elements of Photogrammetry with GIS application. International Edition. Third Edition McGraw Hill, 2004.

[8] R. Frank. History of Aerial Photography: How Photography Shaped Digital Aerial Mapping. http://www.suite101.com/content/history-of-aerial-photography, 2009.

[9] T. Shafiq, A. Stott and J. Hutton. Aerial Mobile Mapping for infrastructure Data Gathering. Map Asia 2008.

2011 IEEE 7th International Colloquium on Signal Processing and its Applications

71