XVIIIth Congress: XVIIIth Congress

kraus, karl; waldhäusl, peter

Bibliographic data

Multivolume work

Persistent identifier:: 1667435949

Title:: XVIIIth Congress

Sub title:: Vienna, Austria 1996

Year of publication:: 1996

Place of publication:: Vienna

Publisher of the original:: Austrian Society of Surveying and Geoinformation

Identifier (digital):: 1667435949

Language:: English

Editor:: Kraus, Karl; Waldhäusl, Peter

Corporations:: International Society for Photogrammetry and Remote Sensing, Congress, 18., 1996, Wien; International Society for Photogrammetry and Remote Sensing

Adapter:: International Society for Photogrammetry and Remote Sensing, Congress, 18., 1996, Wien; International Society for Photogrammetry and Remote Sensing

Founder of work:: International Society for Photogrammetry and Remote Sensing, Congress, 18., 1996, Wien; International Society for Photogrammetry and Remote Sensing

Other corporate:: International Society for Photogrammetry and Remote Sensing, Congress, 18., 1996, Wien; International Society for Photogrammetry and Remote Sensing

Document type:: Multivolume work

Volume

Persistent identifier:: 1667437070

Title:: XVIIIth Congress

Scope:: 230 Seiten

Year of publication:: 1996

Place of publication:: Vienna

Publisher of the original:: Austrian Society of Surveying and Geoinformation

Identifier (digital):: 1667437070

Illustration:: Illustrationen, Diagramme

Signature of the source:: ZS 312(31,B1)

Language:: English

Additional Notes:: Erscheinungsdatum des Originals ist anhand des Copyrightjahrs ermittelt.

Usage licence:: Attribution 4.0 International (CC BY 4.0)

Editor:: Kraus, Karl; Waldhäusl, Peter

Corporations:: International Society for Photogrammetry and Remote Sensing, Congress, 18., 1996, Wien; International Society for Photogrammetry and Remote Sensing; International Society for Photogrammetry and Remote Sensing, Commission Primary Data Acquisition

Adapter:: International Society for Photogrammetry and Remote Sensing, Congress, 18., 1996, Wien; International Society for Photogrammetry and Remote Sensing; International Society for Photogrammetry and Remote Sensing, Commission Primary Data Acquisition

Founder of work:: International Society for Photogrammetry and Remote Sensing, Congress, 18., 1996, Wien; International Society for Photogrammetry and Remote Sensing; International Society for Photogrammetry and Remote Sensing, Commission Primary Data Acquisition

Other corporate:: International Society for Photogrammetry and Remote Sensing, Congress, 18., 1996, Wien; International Society for Photogrammetry and Remote Sensing; International Society for Photogrammetry and Remote Sensing, Commission Primary Data Acquisition

Publisher of the digital copy:: Technische Informationsbibliothek Hannover

Place of publication of the digital copy:: Hannover

Year of publication of the original:: 2019

Document type:: Volume

Collection:: Earth sciences

Chapter

Title:: ON THE AUTOMATED ASSESSMENT OF GEOMETRIC SCANNER ACCURACY R. Seywald

Document type:: Multivolume work

Structure type:: Chapter

ribed an be nt of with ssed. enen er bis auch ieden licher ional d to n the ation inate id y- ative ror is ad by o the tech- / one The ional ogies rinci- ) one hoto- error | and ar ar- tions vaths juare ze of t. In these scanning approaches geometric errors can be produced due to inaccurate assembly of neighboring swaths or tiles. Since it is usually easier to keep the mechanical position- ing error of the sensor low for small scanning regions than for large areas. However different scanning technologies can deliver different types and different quantities of geometric errors. 4 TARGET DESIGN Geometric scanner properties from DTP scanner to the high performance film scanner with geometric resolutions from 400 dpi to 5000 dpi have been investigated using both, glass and film based test targets. As a result of investigations a suit- able high contrast film based geometric accuracy test target is proposed, which allows automated but also interactive geo- metric accuracy evaluation. The aim is to obtain both, global as well as local geometric accuracy out of one digitized tar- get. This leads to the need of an adequate spacing between measuring marks. The higher the density of measuring marks in the target, the better is the ability to detect errors which can be missed otherwise. The dimension of the scanner's built-in CCD arrays leads itself to the limitation of the width of one scanning swath or tile. Currently linear CCD arrays from 300 pixels up to 8000 pixels and square array CCD's up to 2000 x 2000 pixels are used as scanner photo-detectors. Depending on the magnification of the optics different areas on the scanning plane are covered by the photo-detectors. When using a target with a low measuring mark density, pos- sible errors in between two marks cannot be detected and therefore they can be overlooked. An extended number of tests was carried out to determine the feasible number of measuring marks distributed over the test target to produce a sensible and precise geometric accuracy result for various types of scanners. Preferably the target would be available on a glass plate to avoid sensitivity to changes in temperature. However, many scanners are unable to cope with glass plates. Therefore a film based target will need to be available. For film targets it is proposed that a five degree change in temper- ature can cause a change in length of 12 um [Wolber, 1991]. Normally a slight change of distances between marks due to temperature variations does not have an influence on measur- ing accuracy obtained after a Helmert or affine transforma- tion, as long as a global change is affecting the whole target equally. Additionally a general useable target has to be de- signed to support transparent as well as opaque scanning. The marks ought to be symmetric (circles, squares, cross- ON DD Figure 1: A collection of possible measuring marks digi- tized on a DTP scanner. From left to right: circular mark; quadratic mark; cross-hair; circled cross-hair; cross-hair with arrows. hair, see Figure 1) and their coordinates need to be measured on a precision instrument, e.g. an analytical photogrammet- ric plotter. Generally, the shape of the measuring mark de- pends on the type of evaluation process. The circular and square marks are better suited when evaluating by means of a center finding algorithm. Matchig of point positions by 183 correlation is a technique where symmetric marks, especially filled circles or squares are to be preferred. The more com- mon cross-hair is perhaps desired for manual assessments on a computer screen. Former investigations show that measur- ing marks to be centered by a centering algorithm must have a diameter of at least 4 pixels to achieve a centering accuracy which is sufficient [Trinder, 1989] [Maalen, 1993]. This has to be taken into account also when investigating low resolu- tion scanners, where it leads to the need of relatively large measuring marks. It is of main importance for evaluation by an algorithm to ensure high contrast which allows that the measuring marks are distinct from a target's background and therefore can be segmented easily. As a result we have de- signed a target as shown in Figure 2, suitable for the entire range of low resolution DTP to high resolution film scanners, flat-bed and drum devices, opaque and transparent systems. Figure 2: A subsection of the high contrast geometric ac- curacy target containing 1440 (48x30) measuring marks uni- formly distributed over an area of 23cm x 16cm. The distance between neighboring marks is 0.5cm. The dimensions of the quadratic marks are 0.34mm x 0.34mm. 5 VISUAL ANALYSIS The center of a digitized mark has to be found visually by placing a cross-hair cursor at the desired location of the mark. This requires the availability of a zooming function to find the mark with sub-pixel accuracy. It should be possible to de- termine the center manually with an RMS error of typically 5 % of a pixel [Trinder, 1987]. Manual cross-hair pointing precision can deteriorate as target sizes increase. Additional errors can occur due to asymmetric distribution of the pixels on the target and due to low signal to noise ratio (SNR). The geometric accuracy of the scanner is then obtained from computations comparing the measured positions of digitized marks with those known on the analog original. However, due to the significant effort for manually measuring a large number of marks (e.g. 600 marks) on a computer screen it is essential that visual analysis only should support the auto- mated investigation. Support can be needed for verification of unusual, dubious or obviously false results obtained by au- tomatic evaluation. International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996

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