What is spatial information technology and GIS?
Spatial means related to space or location. Spatial information technology is the use of tools like GIS, GPS and remote sensing to collect, store and study data about places.
A Geographical Information System (GIS) is a computer system to capture, store, check, link, analyse and display data that are tied to locations on the Earth. It keeps both where something is (spatial data) and what it is (attribute data).
Components of GIS
- Hardware: computer, storage, scanner, digitiser, GPS receiver, printer/plotter.
- Software: GIS programs to input, store, query, analyse and display data (free ones like QGIS exist).
- Data: spatial data (maps, images, coordinates) and attribute data (tables).
- People: trained users, from technicians to planners, who design and use the system.
- Methods (procedures): the steps and rules for good analysis.
Advantages of GIS
- Handles large data quickly and keeps it updated.
- Can ask questions of the map: "Which villages are more than 5 km from a hospital?"
- Combines many layers (overlay) to find suitable sites.
- Maps can be changed and redrawn at any scale and in any style quickly.
- Links maps with tables, so clicking a feature shows its details.
- Helps in planning, disaster management, agriculture, transport and environment.
Spatial data formats: raster and vector
Raster
A grid of equal square cells (pixels). Each cell has one value (e.g. land use code or height). Satellite images and scanned maps are raster. Simple structure and good for overlay, but edges look blocky and files can be large.
Vector
Features drawn with exact coordinates as points (a well, a town), lines (a road, a river) and polygons (a lake, a field). Sharp boundaries, small files, and easy to link with tables, but analysis can be more complex.
Data input and linking attribute data
Ways to enter data
- Digitising: tracing features from a paper map with a digitiser or on screen.
- Scanning: turning a paper map into a raster image.
- GPS: recording coordinates in the field.
- Remote sensing images and existing digital data.
- Keyboard entry of tables and coordinates.
Linking spatial and attribute data
Each feature gets a unique ID. The same ID is in a row of the attribute table (name, population, water source…). This link lets you click a feature to see its data, or pick rows in the table to see them on the map. Maps must also be put on a common coordinate system (georeferencing) and checked for errors.
Spatial analysis: overlay and buffer
Overlay analysis
Two or more layers of the same area are put on top of each other to make a new layer. For example, flat land overlaid with near a river gives land that is both flat and near water, good for farming. Overlay answers "where are A AND B both true?" (It can also find A OR B.)
Buffer analysis
A buffer is a zone of a fixed distance around a point, line or polygon. Examples: a 500 m zone around a school where no liquor shop is allowed, a 1 km zone along a river for flood risk, a 2 km zone around a factory for pollution check. The buffer can then be overlaid with other layers (e.g. houses) to count what falls inside.
Other analyses include network analysis (shortest route) and query (select by attribute).
Key formulas and definitions
- GIS components: Hardware + Software + Data + People + Methods
- Spatial data = where (location); attribute data = what (details)
- Raster = grid of cells; Vector = point, line, polygon
- Overlay = combine layers (A AND B); Buffer = zone of distance d around a feature
Worked examples
1. A village well is shown by its coordinates. Is it a point, line or polygon?
A point (vector), because it has one location and no length or area at this scale.
2. Which format is a satellite image?
Raster, because it is a grid of pixels, each with a brightness value.
3. A planner wants land that is flat AND within 1 km of a road for a new warehouse. Which analyses?
Make a 1 km buffer around roads, then overlay it with the slope layer (flat areas). The common area is suitable.
4. Houses are at 0.8, 1.5, 2.2, 2.9 and 3.4 km from a factory. How many are inside a 2 km buffer?
Only 0.8 and 1.5 km are ≤ 2 km, so 2 houses.
5. You click on a lake in a GIS and see its name, area and depth. What made this possible?
Linking: the lake polygon has an ID that matches a row in the attribute table with its name, area and depth.
Common mistakes
- Thinking GIS is only software. It needs hardware, data, people and methods too.
- Mixing up raster and vector: raster is cells, vector is points, lines and polygons.
- Thinking a buffer is only a circle. Around a line it is a strip; around a polygon it is a wider outline.
- Overlaying layers that are not on the same coordinate system and scale, so features do not match.