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QUALITY ANALYSIS AND CALIBRATION OF DTP SCANNERS 
Emmanuel P. Baltsavias, Barbara Waegli 
Institute of Geodesy and Photogrammetry, Swiss Federal Institute of Technology (ETH) 
ETH-Hoenggerberg, CH-8093 Zurich, Switzerland 
Commission I, Working Group 5 
KEY WORDS: Scanner, Film, Digitization, Scanner Quality Analysis, Scanner Test Patterns, Scanner Calibration. 
ABSTRACT 
Scanners have been used as input devices in photogrammetric and cartographic applications mainly for digitisation of aerial images and 
maps. This paper deals with the use and applicability of DeskTop Publishing (DTP) scanners for photogrammetric/cartographic 
applications, their quality analysis and calibration. The motivation of the paper is the investigation as to what extent low-priced DTP 
scanners, which are rapidly improving during the few last years, can be used for such applications. The paper will mainly concentrate 
on flatbed scanners with aim the scanning of films. However, many of the topics mentioned in the paper are also valid for drum 
scanners. The paper gives a review of recent technological developments with respect to these scanners, describes advantages and 
disadvantages, presents characteristics and problems of such scanners, and investigations on their geometric and radiometric accuracy. 
Test patterns for calibration of such scanners and a novel, simple and generic geometric calibration method are presented. Results using 
five different scanners show that a geometric accuracy of 4 - 7 uum can be achieved. 
1. INTRODUCTION 
Scanners are an essential component in photogrammetric and 
cartographic applications. They have been used for scanning of 
aerial and satellite images, as well as digitisation of topographic 
and thematic maps, plans, charts and atlases. Aerial and satellite 
imagery has been used to derive Digital Terrain Models, 
orthoimages, and for digital mapping (new generation or update 
of existing map data). A trend is the use of digital orthoimages 
for generation and update of databases, generation of orthoimage 
maps, integration with other raster and vector data and 
visualisation. Although the developments in direct digital data 
acquisition have been enormous in the last decade, film-based 
systems are used in all fields of photogrammetry. In aerial 
photogrammetry film-based systems will provide the main data 
input for many years to come. 
The scanning requirements depend on the document to be 
scanned and the application. Aerial images are scanned in grey 
levels or colour, require a format of 25 x 25 cm, a geometric 
resolution of at least 600 - 1200 dpi, a geometric accuracy of 2 - 
5 um (for high accuracy applications), a radiometric resolution of 
10 - 12 bit and a density range of 2.5 D (panchromatic images) to 
3.5 D (colour images). Satellite images often require a larger scan 
format (up to 30 x 45 cm). Maps/plans pose some additional 
requirements: opaque scanning, large scan format (e.g. A1), 
bilevel and multilevel (usually 1 - 4 bit) and halftone scanning. 
There is no single scanner, that can fulfil all these requirements. 
High-end, A3 DTP scanners come close to fulfilling these 
requirements with the main problems being in the geometric 
accuracy and resolution, and the scan format. 
A classification of scanners is given in Baltsavias and Bill, 1994. 
DTP scanners can be divided in flatbed and drum scanners, or 
low (1,000 - 20,000 SFr. with few exceptions) and high cost (> 
50,000 SFr.). Although drum scanners have a high geometric 
resolution (2000 - 4000 dpi), and high density range (3D - 4D), 
they are generally more expensive than their flatbed counterparts, 
13 
and most importantly they have low geometric accuracy due to 
drum inaccuracies, unflatness of film on drum etc. and because of 
the same problems and the inability to scan glass plates an 
accurate geometric calibration is not feasible. Here, mainly only 
lost cost flatbed scanners will be treated. 
2. OVERVIEW OF DTP SCANNERS 
DTP scanners have been developed for applications totally 
different than the photogrammetric/cartographic ones. However, 
since they constitute the largest sector in the scanner market, they 
are subject to rapid developments and improvements. Flatbed 
scanners typically employ one or more linear CCDs, and move in 
direction vertical to the CCD to scan a document in one swath. 
Usually the stage is stationary, and the sensor/optics/illumination 
move. They can scan binary, halftone, grey level and colour data 
(with one or three passes), may have good and cheap software for 
setting the scanner parameters, image processing and editing, and 
can be connected to many computer platforms (mainly Macs and 
PCs, but also Unix workstations) via standard interfaces. They 
can usually scan A4 format, but some can scan up to A3 or even 
more. Some do not scan transparencies, others do so but only of 
smaller format (for A4 scanners the maximum transparency scan 
width is 8°- 8.5””). Such a width suffices to scan aerial films with 
8 fiducials (5 fiducials are visible). 
Flatbed scanners have a resolution of up to 1200 dpi (21 um pixel 
size) over the whole scan width. Few scanners offer the option to 
increase the resolution by projecting a document portion (smaller 
than the full width) on the CCD. Their price range, with few 
exceptions, is 1,000 - 20,000 SFr. The big price jump occurs 
when going from A4 to A3 format. The transition from 600 dpi to 
1200 dpi costs less. A3 scanners with 600 x 1200 dpi start at ca. 
19,000 SFr. A4 scanners with 600 x 1200 dpi and transparency 
options cost much less (2,000 - 5,000 SFr.). Their radiometric 
resolution and quality, and scanning speed can be comparable to 
or even exceed that of the more expensive photogrammetric film 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996