The question arises as to why we need geographic information systems?
You immediately answer, “To map locations”. The answer is not that simple!
A geographic information system can be used to visualize scenarios, question information, analyze and interpret data to understand trends, patterns and relationships.
GIS Output and Its Types:
- Textual – In the form of tables, numbers, lists or query-generated text.
- Graphical – Consists of maps, graphs, screen displays and perspective plots
- Digital Data – Stored on a disk, tape or communicated using a network.
- Computer-Generated Sound
- 3-Dimensional Images
Do you know bankers use the GIS to track your properties and their values? They employ the text output approach, which is said to be even more important than maps for analyzing data.
The results are compiled in list or table form with a set of features. We can even sift and search through the information or apply mathematical operations on it.
An interesting application of this form of output is the voice generated system in a GPS used in cars.
The tabular mode of representing information is an organized and neat way rather than a list. Different attributes of the items are displayed side by side to contrast and compare.
The list combines all the relevant items and produces them in a single file. Examples include driving directions, ATMs within a 5-mile radius and recreational parks in a city listed in order of size.
You hear the word geography and the first thing that comes to mind is MAPS. The graphical output of a GIS includes maps, diagrams and charts. These graphics allow users to pinpoint objects and easily identify them in their accurate 3-D figures.
Fig 3: A 3-D map
Peripherals are visual displays connected to a system.
Terminals which were television-like started appearing in the 1960s to display graphical output. Nowadays these are widely used as displays in every computer.
Raster and Vector Devices
Graphic output devices are further divided into raster and vector set.
Raster devices use uniform picture elements (called pixels or pels) to fill in the gaps to create pictures. Scanners, Line printers and Dot-matrix printers are some examples.
The resolution of a picture is also expressed in megapels.
A 640×480 pixel resolution = 0.3 megapels.
A 1280×1024 = 1.3 megapels (Just like in a camera)
Vector devices on the other hand draw lines and shade areas to complete the picture. Graph plotter is an example of a vector device.
Vector commands can be used to function raster devices as well. The raster device converts the commands to a suitable display. Raster to vector conversion or vice versa between input and output is applied at a number of points in a GIS.
As you know, line printers are raster devices. They print one line, simultaneously at a time using impact of hammers on an ink ribbon. The first line printers could accomplish 200-1000 lines of text per minute where each line comprised of 132 characters in fixed positions.
A line printer prints sequentially from top to bottom and can only cater to a limited set of characters. The rectangular cells with fixed resolution and locations are a big disadvantage for line printers. Another disadvantage is that it is difficult to draw continuous lines and shades are produced by overprinting thus the results are best viewed from a distance.
Line printers are applied for repeated mapping of data on a constant base e.g. weekly report of rainfall.
Dot matrix Printers/Plotters
A picture is built using rows of dots which are often printed in blocks of 9 or 25 rows at a time. Shades can be printed by changing the fraction of dots randomly printed in a small area. Earlier models used a hammer on ribbon for every dot but recent versions include lasers, Xerox or electrostatic technology producing up to 300 dots per inch.
Fig 1: Dot Matrix Printer Head
Engage a pen moving under computer control to create images. Most of the plotters use a large number of movements in pre-set directions of a specific size. They use separate motors for x and y-axis and smooth diagonal lines can be drawn using a combination of the two motors.
An advantage is this plotter can be used to print on pre-printed base maps so a separate copy of the base map need not be printed.
These scanners produce a soft copy of a hard copy, i.e. a computer image of a physical one. A photographic paper is mounted inside a rotating drum. The image is created using helix-shaped movements of the drum with a light source moving along its axis. Commonly used in remote sensing and image processing. Some devices produce an output directly on a 35 mm slide.
CRTS (Cathode Ray Tubes)
A CRT display is made up of millions of small phosphor dots (in primary colors) that glow. The earliest CRTs had the ability to display rows of characters in a fixed position.
Fig 2: A CRT monitor
Storage Tube Technology
This major breakthrough made in 1970 provided graphic display at a low cost. Images are drawn by a moving electron beam that is computer controlled. The image produced is permanent so all of it must be erased.
Refreshed Image Technology
The feature to refresh the image became available in 1975. Pictures are redrawn from the one stored in internal memory. Lighting dots are used in this technology. Numbers of rows and columns of dots make up the resolution. 1280×1024 is the most common resolution for graphics of high quality.
3 glowing dots representing the 3 primary colors are lit up by different electron guns to create the respective color. The percentage of illumination can be changed to create a variety of colors. The number of colours that can be displayed simultaneously depends on the number of bits per pixel (bit plate). The palette is the maximum number of colours that can be chosen at a time.
Ever seen a 3-D movie? A 3-D display gives us the added dimension of depth by rapid switching between the image for the left eye and the image for the right eye. A filter polarizes the screen and we wear glasses to clear these filters.
Memory and Processing Components of a CRT
- Object memory: To store the group of objects in the image to be displayed.
- Vector-raster converter
- Display memory: The colour number for each pixel is stored
- Colour lookup table: The combination of RGB for each colour in the palate is identified.
- Digital-t0-Analog converter: Converts the digital level to a voltage.
Standards of Graphics
The standard for communicating with most devices is PostScript.