GISdevelopment.net ---> AARS ---> ACRS 1990 ---> Education/Training

Standardization of reproduction of Remote Sensing images and its quality control system

Zhang Qingpu
(Research Institute of Surveying and Mapping)
Beijing, China


Abstract
In this paper based on the principles of color reproduction and spectrophotometry,the distinguishing features of reproduction to remote sensing , geometric of color scanners and the standardization of systematic approach of reproduction are described.

The distinguising features of reproduction to Remote Sensing images1

Over the recent years, cartographers and geographers in the world have produced a lot of photomaps using remote sensing images .Compared with color reproduction in conventional Graphic Arts, reproduction of remote sensing images has its own distinguishing features which may be summarized as follows:
  • The system for reproduction must provide high geometric accuracy.
  • It is important to obtain more image details on printed sheets rather than accurate color rendition.
  • The reproduction approach must have flexibility because of particular requirements for different originals.
The analysis of geometric accuracy of color scanners
It is known that in process of reproduction there are a lot of factors influencing the accuracy of scanning systems, such as lenses distortion, film distortion, magnification, tolerance of the optical and mechanical construction of scanners working conditions and so on. For practical purposes we have confined ourselves to examination of the total accuracy of scanning systems only instead of analyzing every factor separately. The objective of examinations is:
  • To check if scanners have the same distortion in both vertical and horizontal directions.
  • To determine whether the standard deviations of the scanning systems are acceptable.
  • To detect if there is any spherical distortion in scanning systems.
To this end we prepared a line original in polyester base. To determine relationship between magnification indexes of scanning systems and the accuracy of reproduction we scanned the original line at different scanners. Applying the principles of the least square method the accuracy of scanning systems was calculated .The results are shown in Table 1.

Table 1
Scanners Mag. (D) max.(mm) g(mm) SHV dHV
(DH) (DV) gH gV gHV
DC-400B 1 -0.10 -0.07 ± 0.06 ±0.04 ± 0.05 -0.03 ±0.04
2 -0.15 -0.06 ± 0.11 ±0.04 ± 0.08 -0.06 ±0.05
4 -0.22 -0.08 ± 0.17 ± 0.17 ± 0.13 -0.08 ±0.11
6 -0.09 -0.10 ± 0.08 ±0.10 ± 0.09 0 ±0.09
M-550 1 -0.23 -0.09 ± 0.13 ± 0.04 ±0.10 -0.06 ± 0.08
2 -0.29 -0.05 ± 0.26 ± 0.03 ±0.19 -0.12 ± 0.14
4 -0.35 -0.13 ±0.13 ± 0.30 ± 0.09 -0.12 ±0.19
8 -0.73 0.20 ± 0.56 ± 0.05 ± 0.40 -0.22 ±0.33


In Table 1 (D) max maximum error at n measured lines
(D) H-error in horizontal direction,
(D) V- error in vertical direction,
g-root mean square error,
gHV -root mean square error in horizontal and vertical directions,
SHV systematic error in both horizontal land vertical directions,
dHV - Standard error in both horizontal and vertical directions,

After analyzing data in table 1 it can be pointed out.
  1. The vertical and horizontal directions of scanning systems produce different dimensional distortion and the distortion in vertical direction appears quite stable in range of magnification from 1* to 8*
  2. Consideration of tolerance margins in reproduction of remote sensing images the DC- 300 B scanner can provide the acceptable geometric accuracy until 6*, while the M550 scanner can be used till 4*.
  3. No spherical distortion in scanning systems has been found.
Standarization and specfication for reproduction.
In order that a set of original separations and proofs may be produced and can easily be matched it is necessary to specify the following parameters:
  1. Computations of CIE Uniform space coefficients of ink sets-2 The results of measurement are shown in Table-2

    Table 2
    Colours Ds Tristimulus Values Uniform Colour Space
    X Y Z L* a* b*
    Yellow 1.03 62.14 67.28 10.54 85.65 -7.54 85.37
    Magenta 1.40 32.99 18.35 34.40 49.92 64.53 -19.67
    Cyan 1.50 21.80 28.88 7.37 60.68 -26.81 -39.68
    Black 1.62 5.48 5.51 7.10 28.14 1.40 -2.68


  2. The data of colour cast grayness and color efficiency of inks are given Table 3

    Table 3.
    Colours Solid Density Colour
    Cast    		 Grey Efficiency
    R G B
    Yellow 0.04 0.09 1.06 5% 40% 94%
    Magenta 0.20 1.40 0.74 45% 14% 66%
    Cyan 1.54 0.51 0.18 25% 10% 78%


  3. Colorimetric measurements of map Paper The densitometric and spectrophotometric data of coated paper are listed in Table 4

    Table 4.
    Paper Weight
    (g m2)    		 Solid Density Uniform Colour Space
    R G B L* a* b*
    Coated 157 0.07 0.01 0.04 97.26 -1.73 -1.21


  4. Determination of print contrast. The print contrast is limited to 0.3 ±0.03 for yellow, 0.35±0.03 for magenta, 0.35±0.03 for cyan and 0.4±0.03 for black.
  5. Determination of color contrast. It is 70±5% for yellow colour ,50± 5% for magenta and 55% ±5% for cyan.
  6. Determination of optimum ink level. The optimum ink level are defined by the maximum values of print contrast and colour contrast. The control data is 1.05±0.03 for yellow, 1.40± 0.03 for magenta 1.50±0.03 for cyan and 1.60±0.03 for black.
  7. Investigation of dot gain The actual control data of dot gain is 11%±2%.
  8. Printing ink sequence. In our case the printing sequence is black, cyan, magenta and yellow.
  9. trapping determination . the control data is 110%±3% for cyan+ magenta " combination. 105%±3% for "cyan+ yellow" combination, 106%±3% for "magenta+ yellow" combination and 112%± 3% for "cyan+ magenta+ yellow" combination.
  10. Determination of grey balance. The grey balance is determined by either colour atlas or mathematical calculation. In our case the grey balance data are shown in table 5
Table 5
Set A Set B
Density Cyan Magenta Yellow Density Cyan Magenta Yellow
1.37 100 90% 85% 1.33 100 90% 85%
0.79 70% 55% 51% 0.83 70% 62% 55%
0.49 50% 35% 30% 0.59 50% 45% 40%
0.28 32% 20% 15% 0.35 32% 28% 24%
0.14 16% 12% 9% 0.19 16% 14% 12%


Construction of quadrant diagram for reproduction4
After having determined the specification of reproduction, a practical technical approach to reproduction has been set up. We find a quadrant diagram to be a useful tool. This takes account of the specification and influence of viewing conditions on the appearance of transparencies and final printing sheets. The procedure of improved approach can be summarized as follows:
  1. Original analysis and densitometric measurements. It is necessary to analysis the originals and to determine Dmax, Dmin, ?D and D-. After experiments, we put forward the specification of remote sensing image originals.
  2. Determination of the scene density range of the original, using the transparency characteristics ( Dmax = 2.4, Dmin = 0.4, ?D = 2.0) and the characteristic curve 1 ( Fig.1 first quadrant), the scene density range of the original subject can be defined . In our case, it is 1.3.
  3. Determination of the intermediate tone reproduction curve 2 (see Fig.2, quadrant 4). This accommodates the influence of viewing conditions on the appearance of a transparent original and printing conditions.
  4. Definition of tone reproduction curve 3 (See Fig.1 quadrant 2) . For this purpose, the intermediate tone reproduction curve and characteristic curve are applied.
  5. Determination of grey balance curves ( Fig. 3 quadrant 3).
  6. Defining the four separation positives (negatives) required using the grey balance curves and the tone reproduction curve ( Fig. 3 quadrant 1).
  7. Determination of colour correction amount. Initially colour correction is established applying manufacturer's recommendations, not based on any measurements of ink sets.
  8. Platemaking, proofing and printing are carried out by the following specification ff reproduction
Conclusion
  1. Considering the particular requirements, the geometric accuracy of colour scanners is high enough for reproduction of remote sensing images.
  2. The feasibility of improved quadrant diagram approach for reproduction of remote sensing images is adequate to achieve satisfactory results with scientific and industrial characteristics.
  3. The research indicates that the important parameters of reproduction may be measured and hence may be controllable within certain tolerance and that the reproduction quality can be assessed objectively.
References
  1. Sunderland, BHW, TAGA Proceedings, (1979, 67.
  2. Richard E. Maurer, TAGA Proceedings, (1979), 209
  3. Yule J.A.C, Principles of colour reproduction, John Wiley & Sons, Inc. (1967), 84.
  4. Zhang Qingpu, TAGA Proceedings, (1983) 668.