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Mapping from Space
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Using SPOT Image in Topographic Mapping
at Scale 1:50000
Ramin Rahimi Dgafari
National Cartographic Centre of Iran (NCC),
P.O. Box 13185 - 1684 Tehran, Iran
phone + 9821 6000031-9, fax :+9821 6001971
e-mail: ncc iran@irearn.bitnet
Abstract
Nowadays, There is much concern to use SPT Satellite data in topographic mapping specially for those areas where these data are the best solution (like flat desert area with a few features). To do so we should seriously consider to those factors which play some roles in effective use of SPOT data in topographic mapping. In this paper using SPOT images for making topographic maps at scale 1/50000 will be studied from different point of views. Including completeness (Image content) and interpretability; Moreover some accuracy standards, accuracy of SPOT data in topographic tasks, Stereo-measurements and methods to select necessary control points will also be evaluated.
Introduction
Spot I was launched on Feb. 1986 and since then it has been transmitting high quality imagery of the earth back to ground stationsand it has been in a near polar circular orbit, it has 10m. panchromatic pixel size. off nadir viewing capacity, stereo-capability and ability to commission imagery for specific areas and therefore we can say SPOT has characteristics which should be a boon to the cartographic community. There are different cartographic applications for SPOT images for example topographic map inspection, map revision and compilation and generation of digital elevation data. Image quality and geometric accuracy of SPOT data are essential elements in cartographic applications. In this paper applications of ASPOT imagery for topographic mapping at scale 1:50000 will be evaluated.
SPOT data
SPOT data has been available since 1986. We should consider that mapping from SPOT is not simply a matter of tracing off detail from the imagery into the map and the geometry of the new image is both complex and dynamic. The SPOT HRV sensor has an array of 600 charged couple device (CCD) sensors which sample reflected radiation every few milli-seconds to create a digital image swath 10 m. long and 60 Km. wide. Each swath has a perspective but off nadir view and a complete image is built up swath by swath while the stellite rotates around its three axes. SPOT data are recorded by two push-broom scanners abroad the satellite. These scanners can operate in two separate modes: multi spectral (ms) or panchromatic (pan) with 20m. and 10 m. spatial resolution respectively and it should be noted that MS images provide relatively high spectral contrasts while the PAN images have much more subtle contrasts and trade off between them should be taken. Also SPOT sensors can record either vertical or oblique image data and the oblique data can be corrected to produce rectified images. SPOT has different processing levels for different applications. The most basic level is ISA, corrected for sensor radiometric calibration but with no geometric correction. Level 1B data is additionally corrected for known geometric distortions including earth rotation, earth curvature, sensor viewing angle and satellite attitude variations. Level 2 is rectified for a map projection system using ground control points and it does not take into account terrain relief distortions and level S is rectified to a reference scene for multidate studies. There are also some auxiliary data including scene information, sensor data and ephemeris data. There are limiting factors in SPOT data acquisition e.g. clouds, mist, dust... make it difficult to obtain data when and where we want and the lack of ease in identification requires more ground effort.
Interpretability
For making true decisions about using SPOT for topographic maps at scale 1:50000 we should consider to interpretability of SPOT images. Although all of the geographic information required for a 1:50000 topo map are not provided by SPOT but however most high priority map features can be accurately mapped from SPOT images and also field completion and verification are very important for classification and identification of features. Man features are not directly distinguishable on the imagery but their presence is discovered b the context of their surroundings. In general most features especially roads, railways and built up areas are considered easily detected (THIRL WALL, 89) and uniqueness of tone, color, size and/or shape of features are important controlling factors for identification and also other factors are locational setting of features and their interrelationship. In general we can say that line features are very easily detected roads, railways and rivers (THRIL WALL, 89).
The two types of SPOT image are complementary in qualities while MS images have a higher spectral content, PAN images have better spatial information and a balance between these two kinds of images should be taken for each specific project and many interpretation problems can be solved b using MS and PAN images together and it should also be noted that stereo-analysis is not necessary for extraction of geographic features but it assists in clarifying interpretation. SPOT images should be selected carefully for each specific mapping project and is preferable to use images from different seasons and if we can only use one image for mapping, a summer image is recommended, it should be noted that two important factors for interpreting details from SPOT imagery are the quality of imagery and the experience of the operator.
Completeness
Under this title we are considering about image content which defined as the information on a specific set of features. Image content is dependent on many factors particularly it depends on spatial resolution of the image in relation to the size, shape and pattern of the features and also the spectral response of the features as they are expressed through tone or color and contrasts on the image. An experiment has been do in a test area in QUEBEC, CANADA (THIRLWALL, 89) and it has resulted that for both two kinds of SPOT image the information content is very good but the PAN images provide much details and higher spatial resolution which displayed better representation of size and shape than MS images but it does not mean that mapping results use both PAN images and we should use both PAN and MS images with suitable balance together. It should also be said that another test has resulted that the percentage of detail on the 1:10000 scale map plotted from the POT imagery is 92.1 percent and the same for the 1:50000 scale map is 82.3 percent and these figures don't take account of errors of commission which would also need to be edited during field verification (GUGAN & DOWMAN, 88). IT should also be noted that SPOT imagery contains a lot of useful information which can be utilised for map revision purposes when image interpretation is less important than the recognition of areas of changes.
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