GIS stands for Geographical Information System. It refers to a computerized system for data management and serves the purpose of capturing, storing, managing, retrieving, analyzing and displaying spatial information. Spatial information or data refers to any data that is directly connected to a specific location or geographical area. In this blog post I overview four data input techniques in GIS.
GIS uses three types of data:
- Raster data (spatial): refers to discrete cells in a rigid row by column format, e.g. satellite or aerial images.
- Vector data (spatial): refers to features on the surface of the earth represented by points, lines and polygons.
- Attribute data: refers to data that’s attached to the spatial data
Although most GIS systems can display a combination of both types of data, only some GISs can analyse both. Most GIS have several features, including the following:
- It enables the user to input digitized data that is geographically referenced to a certain location.
- It enables the user to use relational database management to link certain attributes to each feature.
- It enables the user to analyze relationships between several geographic features with the help of a vast range of spatial operations and features.
- It enables the user to produce high quality images on color monitors or plotters.
There are Four Data Input Techniques in GIS
Fig 1: The Four Data Input Techniques in GIS
There are four common data input techniques in GIS. These are shown in Figure 1. The choice of mode depends on application, available budget, type and complexity of data.
- (a) Manual digitizing – digitizing tablet:
A digitizer is a special table with a grid of fine wiring behind the face. You place your map on it and secure it in place with removable sticking tape. You then trace the spatial features using a magnetic pen (puck) which sends the data collected to a computer. This data is then registered to a coordinate system based on the bounding coordinates of the map that you enter when you start the digitizing process.
Digitizing using a digitizing tablet can be carried out in the following modes:
- Point mode: single points are recorded at one time.
- Stream mode: one point each is collected at regular breaks of time and distance.
- On-screen mode: when the digitized line-work is displayed on a graphics terminal while being digitized.
Digitizing can be very time-consuming and so expensive. However, if the digitized maps are managed correctly they can be used over-and-over, and so for this reason become cost effective. Digitizing can take time to learn and to master. All I can say about this is “thank goodness that the bulk of this work is complete – its a real pain”!
1. (b) Manual digitizing – onscreen heads-up
A second manual digitizing technique is known as heads-up digitizing. This is where you…
- scan a map to create a digital version of it
- place the scan into a coordinate system. You do this using a GIS process called georectification. Simplistically this involves matching features on the scanned map to identical features on a GIS map.
- Use the georectified scanned map as a backdrop on your computer screen and then trace the features on the scan with your mouse. The features will automatically be in a coordinate system, and so, once attributed, a GIS map.
Sounds simple, but there are many tricks that you can use to shortcut the process while at the same time improving the quality of the end product. Tricks like ensuring you’re using the correct coordinate system, copying features from other GIS maps, setting up your scan so that the georectification process is quicker and more accurate, and giving yourself a tidy edge to digitize against. I show you all these tricks and more using the free open source QGIS. Click this link to view more information about the course and enroll for just $9.99 (normally $39.99)
- Automatic scanning:
This is where a scanner captures the spatial data automatically. Scanning devices can capture spatial data at a very high speed. However, these scanners are often quite expensive and are usually unable to recognize text and symbols. Scanned data also requires a large amount of editing, which is done manually, therefore it can be more time consuming. However, it can be very quick when the map being scanned is a single map theme eg. just roads, as different from a final published map that often contains a mix of themes such as roads, rail, watercourses, etc.
Scanners are expensive and its better to outsource this task. The final scans are only as good as the original map. Low quality scans might need to be on-screen digitized rather than going through feature auto-recognition.
3. Entry of coordinates via coordinate geometry:
Coordinate geometry is a means of entering explicit measurements of features from some known monument. This data is entered based on surveys. It involves manually entering X & Y coordinate information, and distances, can be very expensive in terms of labor. It is hardly used for natural resource applications in GIS and is mostly used for producing detailed maps of property.
- Conversion of existing digital data:
This involves the process of converting data from CAD systems to a GIS or topological form. Most software vendors of GIS provide a data exchange format where users can write their own data conversion routes.
The GPS is also another means of data conversion where spatial features are represented in 3D maps to direct routes to reach a certain location.
There are four main data input techniques in GIS. Which method you choose will depend a great deal on the volume of digitizing you have to do, and the format that your data is in. Each method has its own advantages and disadvantages which clearly differentiate one from the other. It is up to the user to choose any mode of data input based on his requirements. For the specialist projects I work on I mostly use the on-screen digitizing technique.
You might also be interested in this post about GIS data sources