ACCURACY OF MAP DIGITIZING BY USING DIGITAL CAMERATakashi YAMAMOTO*, Jong-Hyeok JEONG*, Masataka TAKAGI*, Masataka TAKAGI*
Kochi University of Technology*

ABSTRACT: In the past, a large accurate image scanner was used for map digitizing. The image scanner isso expensive that it is difficult to use in general. Nowadays, digital camera with 10 mega pixels can be usedin many fields. In the future, such digital camera will be able to replace the image scanner. Therefore, it isimportant to consider an accuracy of map digitizing by using digital camera. In this study, paper map wasprepared for the experiment. In the paper map, four control points and two validation points were allocated.
The paper map with transparent plastic board was taken by the digital camera. Coordinates of the control
points were extracted 20 times by visual interpretation. And coordinates transformation was established bytwo-dimensional projection transformation. Then coordinates of validation points can be calculated by thetransformation. And this calculated coordinates of validation points were compared with true points. As aresult, coordinate of validation points had approximately 1mm error. If distortion by the transparent plastic
board is eliminated, the accuracy will be improved.
KEYWORDS: map digitizing, digital camera, two-dimensional projective transformation
1. BACKGROUND

points.

In the past, a large accurate image scanner was used
for map digitizing. The image scanner is so

3. CAMERA SELECTION FOR DIGITIZING

expensive that it is difficult to use in general.

In this study, the compact type digital camera wasNowadays, a digital camera with 10 mega pixels canselected. Table 3.1 shows comparison between

be used in many fields and it can be obtained

characteristic between single-lens reflex type digitalcomparatively cheaply. In the future, such a digital

camera and compact type digital camera. Thecamera will be able to replace the image scanner. If

compact type digital camera has a longer range insuch a digital camera can be used as the image

focus than the single-lens reflex type digital camera.

scanner, the integration of GIS data will be improved. Therefore, the compact type digital camera was
Therefore, it is important to examine the accuracy ofmap digitizing by using a digital camera.
2. OBJECTIVES

suitable for map digitizing to keep good focus.
Table3.1 Comparison table of digital camera type
Characteristic

In this study, the accuracy of map digitizing by using

Size of CCD

digital camera will be discussed. Geometric

Length of lensSingle-lensreflexCompact

Large

SmallLong

Short

transformation is established by two dimensional

focus

projective transformation. The accuracy of map

Depth of focus

ShortLong

digitizing is assumed an extraction accuracy of target

Depth of field

ShortLong4. TEST MAP

equation (equation (5.1)).

4.1 Test Map
The 1/25,000 map was assumed in map digitizing sothat the test map was A3 size paper in this study. In
the test map, four control points (C1, C2, C3, C4)
and two validation points (V1, V2) were allocatedusing CAD (figure 4.1).

where u and v are camera coordinates of control
points and validation points; f is the focal length ofthe digital camera; X, Y and Z are the coordinates of
Figure4.1 Control points and validation points incontrol points and validation points in object space;the paper map

X0, Y0 and Z0 are the position of the digital camera
in object space; ω, φ and κ are the attitude of

4.2 Figure of control points and validation points

the digital camera;

Figure 4.2 showed distribution of control points andTo simulate the error of camera coordinate of thevalidation points. The cross figure was used and thecontrol point, random numbers in normal

size is 5.65mm by 5.65mm. Coordinates of control

distribution are generated by using Box-Muller

points and validation points was established as

method, and then they are added to the camera

center of the cross figure.

coordinatesof the control points. At this time theerror of

the control point is 0.5 mm in standard

deviationand the error of the camera coordinate of

the control point is 0.5 pixel in standard deviation.
Thecamera coordinates of validation points

Figure4.2 Figure of control points and validation

includingpointstwo-dimensional projective

error can be calculated usingtransformation(equation (5.2)) from the camera
5. EVALUATION BY A SIMULATION

coordinates of the control points including theThe extraction accuracy of target point can beerror.

evaluated by simulation.
5.1 Simulation Methodology
First, the camera coordinates of control points andvalidation points are calculated using collinear

where x and y are the coordinates of control points

and validation points in object space; u and v are
the camera coordinates of control points andTable5.1 Average and standard deviation ofvalidation points; a1, a2, a3, a4, a5, a6, a7 and a8

calculated error in object space by the simulation

are transformation coefficients. Figure 5.1 showed(units : mm)flow of the simulation.
5.2 Simulation ResultPoints name

Error average

Error SD

V1

0.483

0.227

V20.467

0.225

Table 5.1 showed average and standard deviation ofcalculated error in object space by the simulation.
The result showed enough accuracy for several
purposes as GIS data.

6. METHODOLOGY
6.1 Camera Calibration
As a practical matter, the image which was taken thetest map must be calibrated using equation (6.1).
Center point of lens distortion was assumed as center
point of the image. After that control points wereextracted using calibrated image.

where Δx and Δy are revision value; x and y arecamera coordinates of each pixel; xp and yp are
center point coordinates of the image; k1 and k2 arelens distortion coefficients of radiation direction.
6.2 Conditions in the Experiments
The test map was taken by PowerShot A640 which isthe compact digital camera with 10 mega pixel.
Table 6.1 showed condition of digital camera when
the test map was taken. The digital camera wasmounted on a pedestal. Range from CCD of thedigital camera to the test map is approximately 80
centimeter and test map is about parallel to thedigital camera. The test map was covered 1Figure5.1 Simulation flow

millimeter thickness transparent plastic board topress wrinkle. Condition of the digital camera wassame condition with lens calibration. Focus wasinfinity, and F value was maximum to keep good
focus condition. Table 6.1 showed conditions in theexperiments.
Table6.1 Specification of Camera
Item

Value

Pixel number

3648*2736

F value

8.0

Focus

Infinity

Length offocus lens

7.3mm

6.3 Establishing Transformation
Camera coordinates of control points and validation
points were extracted 20 times by visual Table 6.1
Specification of camera interpretation. The twodimensional projective transformation wasestablished by least square method with 4 control
points. After that coordinates of validation point inobject space were calculated in object space. Thencalculated validation point coordinates werecompared with CAD data. Table6.2 showed
experiments flow.

Figure6.2 Experiments flow
7. METHODOLOGY
Table 7.1 showed standard deviation of camera
coordinates which were extracted 20 times. The
standard deviations showed less than simulation.
Figure6.1 Condition in the ExperimentsTable7.1 Standard deviation of camera coordinates
Points nameSD(pixel)

C10.100

C20.040

C30.083

C40.105

V10.063

V20.092

digital camera such as more control points, camera
calibration that correspondence to various focus andvarious position of the digital camera.

REFERENCES
N. Imoto, S. Hattori, K, Akimoto, Y. Oonishi, 2003.
Camera calibration in twodimensional target field,

Table 7.2 showed average and standard deviation oferror in validation point in object space byexperiments. The average of calculated error by2003 annual academic lecture memoirs of Japan
Society of Photogrammetry and Remote Sensing.
(Lecture Memoirs, In Japanese)experiments shows bigger than simulation. But
calculated error in standard deviation by experimentY. Eshima, H. Tikatsu, 2003, The accuracy
inspection of card size digital camera on digital

showed smaller than simulation.

photogrammetry, 2003 autumn academic lecture
Table7.2 Average and standard deviation ofcalculated error in object space by experiments

memoirs of Japan Society of Photogrammetry andRemote Sensing, pp.63-66. (Lecture Memoirs, InJapanese)(units: mm)
Points name

Error average

Error SD

V1

1.023

0.046

V20.844

0.049

K. Yada, M. Tokunaga, 2005, Target manufacture foraccuracy inspection of digital camera image,Geoinformation Forum Japan 2005 student forum
announcement memoirs Vol.7, pp.111-112.

Covered plastic board might make distortion in the(Announcement memoirs, In Japanese)average because of refractive index.
T. Ootake, H. Tikatsu, 2005, Camera calibration
8. CONCLUSIONS
Map digitizing by using digital camera with 10 megapixels can extract the coordinates of target points inobject space with approximately 1mm error inmethod by Image Based Integrated Measurement,
Geoinformation Forum Japan 2005 student forum
announcement memoirs Vol.7, pp.121-126.
(Announcement memoirs, In Japanese)standard deviation. The average of calculated error
in object space by experiments is bigger thansimulation and the calculated error in standard
deviation by experiments is smaller than simulation.
As a result, refractive index of plastic board should
be considered. Extraction accuracy of target point byexperiments was examined in certain same situation.
Therefore, various camera situations should beconsidered for practical map digitizing by using

Y. Eshima, H. Tikatsu, 2005. Camera calibration
method by using triplet images,
URL:http://www.chikatsulab.g.dendai.ac.jp/sforum/pdf/2005/34 ohdake.pdf, TOKYO DENKI
UNIVERSITY Graduate school of Science andEngineering(last data accessed: 26 Dec 2006).
(Website Reference, In Japanese)Y. Hashidume, H. Tikatsu, 2006, Relations withinside rating element and measurement accuracy of
consumer digital camera, 2006 autumn academic
lecture memoirs of Japan Society of
Photogrammetry and Remote Sensing, pp.141-144.
(Lecture Memoirs, In Japanese)
Y. Kunii, S. Murai, 2006, Consider of ideal situationin calibration of digital still camera, 2006 autumn
academic lecture memoirs of Japan Society of
Photogrammetry and Remote Sensing, pp.145-148.
(Lecture Memoirs, In Japanese)