G. PETRIE Department of Geography University of Glasgow Glasgow G12 8QQ, Scotland, U.K. Hardware Aspects of Digital Mapping* Digitizing units, hardware interfaces, graphic displays, and interactive graphic systems are described, and the problems of cost, software needs, data base or information system definition, and implementation in developing countries are outlined. DIGITIZING OF PHOTOGRAMMETRICALLY-PRODUCED GRAPHIC PLOTS N THE FOUR-YEAR PERIOD since the last I.S.P. I Congress at Helsinki, a large number of map- The philosophy of those mapping organizations ping organizations have either entered the digital which avoid digital photogrammetric mea- mapping field or have developed their existing surements can perhaps be exemplified by the capability substantially. Indeed, there are few Ordnance Survey, which has a large and well- agencies of any size in the more highly developed developed digital mapping system (Thompson, countries that do not have some digital mapping 1979). The basic mapping scales for Great Britain capability, however limited or experimental this are 1:1,250, 1:2,500, and 1:10,000. Up to the pres- ABSTRACT:An introduction sets forth the respective merits of (I) digitizing photogrammetrically-produced graphic plots and (2) direct photogrammetric digitizing. The characteristics of the various types of photogrammetric digitiz- ing units are then discussed, including representative examples of large multi- station systems. With the increasing use of computer-based digitizing units, there has been a strong and welcome trend towards the implementation of standard hardware interfaces which allow designers a wide choice of peripheral devices to use with these units. Another development has been the incorporation of graphic displays into photogrammetric digitizing systems. While vector- driven storage tubes are most commonly used, the recently deueloped raster- driven refresh tubes show much promise for the future. A summary account is given of the experiences with interactive graphic systems interfaced directly to stereoplotting machines reported by a number of North American mapping agencies. In the general conclusion. the opinion is expressed that the hardware f& photogramm~trically-derioeddigital happinghas been deueloped at such a speed recently as to outpace the abilities of many users to be able to implement it fully. A final cautionary note is ma& regarding the dqficulties of imple- menting high-technology digital mapping systems in developing countries. may be. It must be recognized however that, in ent time, most of the O.S. digital mapping activity quite a number of cases, this activity is based on has been concentrated on the 1:1,250scale, which the digitizing of graphic documents, even though covers urban areas, and on the 1:2,500 scale, these are often produced photogrammetrically in which covers the most highly developed rural the first instance. areas. Both series are planimetric only, i.e., they * Paper, Commission IV, 14th Congress of the are uncontoured. At these large scales, there is International Society for Photogrammetry, Hamburg, much and the 13-25 July 1980. photogrammetrically-plotted data, for example, PHOTOGRAMMETRICENGINEERING AN& REMOTE SENSING, Vol. 47, No. 3, March 1981, pp. 307-320. PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1981 the setting back of the plotted roof lines of build- be presented to a photogrammetric audience, it ings to give correct ground lines, the measurement will concentrate on those operations which are of features obscured by trees and vegetation, etc. mainly photogrammetric in character and, in par- (Cardiner-Hill, 1974). As much as 25 to 30 percent ticular, on the operations necessary to collect the of the detail in a single map sheet in urban areas basic digital data. Also, to keep the subject area of may be of this altered or supplemental character. coverage within manageable proportions, this Thus, it is argued that it is better to digitize only paper will confine itself to systems based on when a thoroughly field-completed and checked analog stereoplotting machines. Thus, analytical document is available, rather than attempt to make plotters will not be considered except in a single the extensive alterations and additions to digital instance. photogrammetric data via an interactive editing process. Obviously, having a fully annotated, class- ified, and complete map does simplify the dig- There are a large number of digitizing systems itizing process. On the other hand, it also means an which can be attached to an analog type of enormous duplication of measurement, with first stereoplotting machine. Traditionally, these com- the basic plotting carried out in the stereoplotting prise measuring devices (linear or rotary encoders) machine and then later the digitizing process car- mounted on the cross-slides of the machine model ried out on a graphics digitizer. Furthermore, space with electronic units for the decoding, dis- there will almost certainly be a loss of accuracy in play, and output of the measured coordinates and digitizing the graphic document as compared with the control of the digitizing operations (Petrie, that of the original photogrammetric mea- 1972). Increasingly, however, these specially- surements, which may be significant if the data is built hard-wired electronic devices are being re- also required for a digital data base of terrain in- placed by units which make use of computer formation. technology in the form of a microprocessor, mi- The hardware for graphics digitizing has under- crocomputer, desk-top computer, or minicomputer. gone very considerable development over the last four years. For manual digitizing, solid-state tab- lets based on a variety of measuring principles These are still produced in some numbers, have almost entirely eliminated previous designs mainly by the manufacturers of photogrammetric using cross-slides. Also, the first really effective equipment. Wild's EK-12, 20, and 22 units; the semi-automatic line-following digitizer, the im- Zeiss Oberkochen Ecomat 12; and the Zeiss Jena pressive Laser-scan Fastrak (U.K.), has been in- Coordimeter F typify this traditional approach. troduced into several mapping agencies. Finally, The pulse counting and display, mode selection several new fully automatic raster-scan digitizers (point, time, and distance), the setting of coordi- have been introduced, notably the MBB Karto- nate values, etc., are all carried out using scan, which has an array of high-resolution photo purpose-built electronic components. The more diodes mounted on a cross-slide which traverses sophisticated of these units incorporate scalers the map laid out on a flat-bed, so converting it into and transformation circuitry to allow the display of the form of digital data. terrain coordinates and sometimes digital "Dla- nimeters" to give the length of line an2 the area DIRECT PHOTOGRAMMETRIC DIGITIZING covered during measurement. Although a few At smaller scales and in less complex and devel- new units have appeared in the last four years oped terrain, a very much lower proportion of the such as the Logik 5000 (from Denmark), develop- total map information will be produced by field ment in this area is really at a standstill, in that no completion and from other sources, and by far the new concepts have appeared for some time. greatest proportion will have been measured Nevertheless, one must not overlook the fact photogrammetrically. Given, too, the much more that a good deal of the basic data collection for efficient hardware which is now available for in- digital mapping systems is still being carried out teractive editing, the merging of digital data from using these hard-wired units to which a simple different sources Doses fewer and somewhat less data recording device is attached. An example is severe problems than was the case previously. the U.S. Forest Service automated mapping sys- Thus, many more mapping organizations have tem implemented at its Geometronics Service been willing to implement direct photogrammet- Center (Charnard, 1979). Machines as varied as the ric digitizing than before. Stereoplanigraph, Topocart, Stereosimplex IIC, This paper will concentrate on the hardware as- Planitop F2, Kelsh Plotters, and the SMG 410 ap- pects of digital mapping and not the more limited proximate instrument are interfaced to individual and specialized collection of digital height values Altek AC74 digitizing units which perform off-line for a DTM (Digital Terrain Model) or DEM (Dig- data collection. These data are later edited on M & ital Elevation Model) or for controlling the pro- S digitizinglediting stations with final output on a duction of orthophotographs. Since the paper is to large Kongsberg 5000 automatic coordinatograph. HARDWARE ASPECTS (3F DIGITAL MAPPING A similar situation is reported by the U.S. the microprocessor should otherwise be used. Geological Survey (McEwen and White, 1979), Thus, the ER 34 features the real-time display of which uses Altek AC 189 digitizing units attached terrain coordinates, area measurement, and an in- to Wild B8s and Kern PG-2s for the stereoplotting genious algorithm for data collection, while the component of its Digital Cartographic Applica- PDS-M80 has built-in programs for relative and tions Program (DCAP). absolute orientation as well as the real-time dis- play of ground coordinates. FIRMWARE-BASED UNITS Still more elaborate and expensive units have By contrast with the situation regarding hard- appeared, the best-known being the Kern DC-2B