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GIS and scanning technology

Tamal Pal
GIS Development

Scanning coverts paper maps into digital format by capturing features as individual cells, or pixels, producing an automated image.

Maps are generally considered the backbone of any GIS activity. But many a time paper maps are not easily available in a form that can be readily used by the computers. Most of the paper maps had been prepared on the basis of old conventional surveys. New maps can be produced using improved technologies but this requires time as it increases the volume of work. Thus, we have to resort to the available maps. These paper maps have to be first converted into a digital format usable by the computer. This is a critical step as the quality of the analog document must be preserved in the transition to the computer domain. The technology used for this kind of conversions is known as scanning and the instrument used for this kind of operation is known as a scanner. A scanner can be thought of as an electronic input device that converts analog information of a document like a map, photograph or an overlay into a digital format that can be used by the computer. Scanning automatically captures map features, text, and symbols as individual cells, or pixels, and produces an automated image.

Working of a Scanner
The most important component inside a scanner is the scanner head which can move along the length of the scanner. The scanner head contains either a charged-couple device (CCD) sensor or a contact image (CIS) sensor. A CCD consists of a number of photosensitive cells or pixels packed together on a chip. The most advanced large format scanners use CCD’s with 8000 pixels per chip for providing a very good image quality.

While scanning a bright white light from the scanner strikes the image to be scanned and is reflected onto the photosensitive surface of the sensor placed on the scanner head. Each pixel transfers a graytone value (values given to the different shades of black in the image ranging from 0 (black) – 255 (white) i.e. 256 values to the scanboard (software). The software interprets the value in terms of 0 (Black) or 1 (white), thereby, forming a monochrome image of the scanned portion. As the head moves ahead, it scans the image in tiny strips and the sensor continues to store the information in a sequential fashion. The software running the scanner pierces together the information from the sensor into a digital form of the image. This type of scanning is known as one pass scanning.

Scanning a colour image is slightly different in which the scanner head has to scan the same image for three different colours i.e. red, green, blue. In older colour scanners, this was accomplished by scanning the same area three times over for the three different colours. This type of scanner is known as three-pass scanner. However, most of the colour scanners now scan in one pass scanning all the three colours in one go by using colour filters. In principle, a colour CCD works in the same way as a monochrome CCD. But in this each colour is constructed by mixing red, green and blue. Thus, a 24-bit RGB CCD presents each pixel by 24 bits of information. Usually, a scanner using these three colours (in full 24 RGB mode) can create up to 16.8 million colours.

Nowadays a new technology: full width, single-line contact sensor array scanning has emerged in which the document to be scanned passes under a line of LED’s which capture the image. This new technology enables the scanner to operate at previously unattainable speeds.

Types of Scanners
There are several different types of scanners performing the same job but handling the job differently using different technologies and producing results depending on their varying capabilities.

Hand-held scanners although portable, can only scan images up to about four inches wide. They require a very steady hand for moving the scan head over the document. They are useful for scanning small logos or signatures and are virtually of no use for scanning maps and photographs.

Hand held Scanner

The most commonly used scanner is a flatbed scanner also known as desktop scanner. It has a glass plate on which the picture or the document is placed. The scanner head placed beneath the glass plate moves across the picture and the result is a good quality scanned image. For scanning large maps or toposheets wide format flatbed scanners can be used.
Flatbed Scanner

Then there are the drum scanners which are mostly used by the printing professionals. In this type of scanner, the image or the document is placed on a glass cylinder that rotates at very high speeds around a centrally located sensor containing photo-multiplier tube instead of a CCD to scan. Prior to the advances in the field of sheet fed scanners, the drum scanners were extensively used for scanning maps and other documents.
Drum Scanner

Finally, there are the Sheet fed scanners which work on a principle similar to that of a fax machine. In this, the document to be scanned is moved past the scanning head and the digital form of the image is obtained. The disadvantage of this type of scanner is that it can only scan loose sheets and the scanned image can easily become distorted if the document is not handled properly while scanning. However, the new generation of the wide format sheet fed scanners has overcome this problem and have become indispensable for scanning maps, imageries and other
large sized documents.
Sheet fed Scanner

General Feature of a Scanner
The general features affecting the performance of a scanner are its speed, resolution and the type of interface.

The speed of a scanner is dependent on the size of the document scanned. Larger the document, longer the time it takes to scan. Similarly, resolution also affects the scanning speed. Scanning in high resolution is slow and requires much greater time than scanning in low resolution. Resolution is a very important property of a scanner as well as a scanned image. It is the degree of sharpness of a displayed character or image. For scanners, resolution is generally expressed in dots per linear inch. Thus, 300 dpi means 90,000 dots per square inch. Generally, increasing the scan resolution means increasing the size of the image. Large images, in turn, means larger memory consumption. Hence a trade-off must be maintained between image resolution and image size. In the figure the first image is directly scanned at 90 dpi and the second image is scanned at 270 dpi but has been resized to 33% of the scanned portion. This gives us an idea about the increase in the size of the image with an increase in the size of the scanned image.

A scanner interface is a software that lets the scanner hardware communicate with the application that initiates a scan process. Scanners are generally available in two interfaces:
Small Computer System Interface (SCSI) scanners help in fast scans. The technically advanced scanners usually have this interface. Parallel Interface is used by some low-end scanners. These are slower than the SCSI interface scanners.
Types of Scanning
Scanning captures map features, text, and symbols as individual cells, or pixels, and produce an automated image. Based on the document to be scanned there are different scanning procedures followed.

Black and White Raster Scanning:

Image scanned in B&W
Image scanned in B&W Black and white or “binary” scanning is the simplest method of converting any document and can be performed on line drawings, reduced media, text or any one colour document. This is the appropriate solution for archiving and storage projects, in which the documents will be viewed and printed but never changed. It is, therefore, an ideal solution as the first stage in a planned document conversion project.

Applications

  • Archival Drawing Libraries
  • ElectronicDocument Distribution
  • Vectorization Templates

Drawings can be converted into image files for quick and inexpensive library access. A problem arises, however, when the original drawings are of poor quality. When a document is scanned, imperfections such as background, dirt, residue, or stray markings on original source documents are introduced and stored along with original drawing content. In addition to reducing legibility, these imperfections can enlarge the file size, often by a factor of two or three times. A raster clean-up process electronically removes much of the background “noise” and “dirt” that are contained in poor quality source documents. CleanFile results in files that are smaller and easier to store and retrieve, and reduced media storage costs.

Grey Scale and Colour Raster Scanning:

Image scanned in greyscale
Image scanned in color Gray scale and (especially) colour images can be quite large. It must be made sure that the system is capable of handling files whose size is often measured in tens of megabytes. Because virtually every pixel is populated with a value, an attempt to compress the file results in little or no reduction in file size.

Grey scale or colour scanning is most commonly used for:

  • Loading background images into high-end drawing or mapping software as an information base for advanced project work.
  • Capturing images for use in desktop publishing applications.
  • Frequency analyses of the colour ranges- particularly for infrared and vegetation photos

Applications

  • Aerial photography
  • Toposheets
  • Navigation charts (air and nautical)
  • Full colour maps
  • Brochures and artwork
  • Cartographic base data for “high-end” mapping systems

Sometimes only select information needs to be collected from source documents such as toposheets and other colour originals. The selected information may include contours, hydrology, oil and gas fields, and transportation networks.

Rather than using black and white printing plate separates, separate images of map features are created which can be distinguished by colour. For example, elevation contours can be extracted from a colour image of a toposheet. This process is much faster, and thus more cost effective, than attempting to capture data directly from the colour image.

We can also leave the colour image to be used as a visual background reference or simply as archived information. The resulting file is much smaller and more manageable than the image containing all the colours found on the source document.

Applications

  • Contour maps
  • Road maps
  • Hydrological maps
  • Environmental maps
  • Oil and gas mapping

Processing of Scanned Document
Scanning results in conversion of the image into an array of pixels thereby producing an image in raster format. A raster file is an image created by a series of dots (called “pixels”) that are arranged in rows and columns. A scanner captures the image by assigning a row, a column, and a colour value (black or white, a grey scale, or a colour) to each dot. A continuous image is “painted,” one dot at a time, one row at a time. An associated concept in raster scanning is “resolution”. Most scanning of large format documents is done at resolutions of between 200 and 500 dpi (dots per inch). While higher resolutions create higher quality images, increasing resolution increases file size, often substantially. Increasing resolution from 200 to 300 dpi will increase the file size not by 50% but by 125%, from 40,000 to 90,000 pixels per square inch. A grey scale scan requires more storage than black and white at the same resolution, and a colour image needs even more.

However, most of the GIS applications are based on vector technology, so vector formats are the most common. In vector format, the position of a line is represented by the co-ordinates of the starting and ending points of the line. One of the simplest methods to obtain a vector format of a map or image is manual digitization of the image. Digitization involves tracing features of a source map using a pointing device (called a digitization cursor). The system converts the position of the cursor into a digital signal which can be indexed to show the actual co-ordinates of the point. But this is very time consuming and involves a lot of patience as well as hard work. On the other hand, the scanned image can easily be changed into vector format by heads up digitization in which the operator uses a mouse to interactively edit and clean the raster image and to remove stray marks or line gaps picked up in the scanning process. Further tools allow the user to select individual raster features for vector conversion, invoke automatic line following and thinning vector conversion, direct keying of attribute data, and other tools to speed the process of vector conversion.

Digitizing Table

Choice of Scanning or Digitization
A decision has to be taken over the type of data model to use- raster or vector. However, the choice of data structure for use to any particular application is often an arbitrary decision, since GIS software will generally support both the structures. Data structure is a logical arrangement of data in a format suitable for the system to manage it. Whichever model and structure is chosen, data needs to be converted into a format which can be used by the GIS. Converting data into digital format is a labour-intensive activity, and can account for up to 80% of the total system cost.

Scanning offers ease and speed, but the resulting raster images lack the intelligence needed for vector-based GIS. A fair degree of operator expertise is also required, and compression techniques will need to be applied to keep the files to a manageable size. Vectorisation can be applied automatically or interactively to produce intelligent vector files.

Table digitizing has the advantage of employing inexpensive digitizing equipment. However, operator training is needed to obtain good results. Conversely, the procedure is laborious, time-consuming and, hence, costly.

Other possibilities such as raster-to-vector conversion and pattern recognition are worth considering in this trade-off between productivity, cost, quality and usability. While scanning and table digitizing will accommodate the bulk of conversion needs, from text documents to line art and even video images, special techniques have been developed to enter material from other sources. These range from simple programmes that facilitate the keyboard entry of survey co- ordinates to techniques that reconcile aerial photographs with base maps. Photogrammetric, remotely sensed and CAD-generated data represent yet further potential input sources.

Accuracy of Scanned Images
Scanned images have become the primary source of input data for GIS and thus, the increase in the use of scanners in the GIS environment has compelled us to think about the limitation of the scanners in the form of accuracy of the scanned images. This accuracy of the input data has to be quantified before the user uses it as most GIS software have very specific accuracy requirements. In general, the average GIS database will require that input data be accurate to at least 0.018″. This means that an input data location must be within 0.018″ of its actual geographic location at the scale of the map. Thus, a scanner cannot produce more positional accuracy error than the maximum error allowable in the GIS. Standard accuracy issues such as media stability, source availability, and differences in data collection procedures can be easily quantified and the user can decide whether the resultant data are acceptable for their GIS prior to integration. Now, with the recent influx of scanned data, there is a new issue to be dealt with: the accuracy of the input scanner. Since scanners still tend to be quite expensive, the impact of scanning large amounts of data that do not meet the accuracy requirements of the GIS can be devastating. Users must be able to measure the accuracy of their own scanner and service bureaus must be able to prove scanner accuracy to their clients. Accuracy can be defined as the ability of the scanner to produce an image with output dimensions that are exactly proportional to the input document. The scanned image can be dimensionally correct within the specified tolerances, but nothing can be said about the data within the body of the image. While the image may have exactly the right amount of pixels, features within the image may be as far as three or four tenths of an inch from there correct location at the scale of the map, even though the scanner is operating within stated accuracy specifications. Three tenths of an inch can translate to several hundred metres of error on the ground, depending on the scale of the source map. This is generally unacceptable for any GIS. Thus, it becomes necessary to have an idea about the accuracy of the scanned image so that corrective measures can be easily incorporated in the analysis.

Scanning Products
Production of wide format scanners primarily for use in the GIS industry is the main aim of several major international companies. A brief review about the products offered by these companies:

Contex Scanning Technologies Contex Scanning Technology provides a large range of wide format colour scanners. Some of the features of the wide format scanners are: It comes with a unique CADImage/SCAN+FEATURE Software that provides a host of features like Colour Feature Extraction, Rotation, Alignment, Cropping, Despeckling, Holefilling, Reversing, Mirroring and much more. This gives the flexibility to scan any specific resolution to suit the needs of the software or printer that is used.

The Contex range of scanners is marketed in India by CADD Centre Scanning Technologies-India’s No.1 Wide Format Scanner Company, the Exclusive Distributors of Contex A/S, Denmark-the world’s largest manufacturers of full scale monochrome and colour scanners. More information about the scanners and the range of scanners available can be obtained from https://www.caddcentreindia.com/

Abakos Digital Images Abakos brings the Deskan range of colour scanning systems for users requiring a highly accurate large format colour scanning capability. The important features of the Deskan range of scanners include: Powerful in-built Raster Editing Capability (cost effective raster to vector conversion). OCR (text recognition), manipulation and editing along with Auto scan function for faster scanning. The system is light, compact and easily transportable.
For more information about local distributors visit:

Vidar Systems Corporation VIDAR offers a full line of high-quality, large-format scanners designed to meet the needs of GIS professionals, and reprographics industry. Every scanner includes VIDAR’s TruInfo – a scanner control and archiving software which allows to capture quality images, while quickly and efficiently indexing, organising, and sharing scanned documents. Other features include: Lockheed Martin’s neural-network based AUTOGRAPHICS® software which allows the users never to digitise with a cursor. VIDAR’s exclusive colour separation software allows to extract specific colours from scanned images and build layers of information for use in systems such as Autodesk and ESRI.

The VIDAR range of scanner is marketed and distributed by DIGITAL Electronics, and more information about the scanners can be obtained from

Anatech Scanners Action Imaging Solutions brings out the Anatech scanners which scan maps, colour schematics, drawings, photographs and other opaque colour media. The patented pinch roller document transport technology protects against document damage and ensures accuracy by minimising skew effects. Features include: All digital, low noise design for colour fidelity along with onboard, real time colour classification scheme and a SCSI-II interface. Also an internally isolated optics use patented 3-point mounting for stability and accuracy.
For more details contact: https://www.anatech.com/

Widecom Scanners Widecom is one of the leading suppliers of high performance wide format scanners. The main features of the scanners are: It can scan documents up to a 1/2-inch thick at high speeds and items such as foamboards, artwork, and other unusually thick documents can be digitally saved. WIDECOM’s advanced filters such as despeckle, deskew, diffuse, sharpen, smooth, individual RGB adjustment, and Gamma Correction help in providing better scanned documents. It works in High Speed SCSI Interface. It can also scan documents with a scan area of 72″ (1828mm) wide by any manageable length.
Further information can be obtained from: https://www.widecom.com/