Data input techniques in GIS

Estimated reading time: 6 minutes

Introduction

Here’s an overview five GIS data input techniques (ie. five ways to create a GIS map).

The five GIS data input techniques

There are five common GIS data input techniques. Which technique you choose will depend on the application, your budget, and the type and complexity of the data involved.

1. Digitizing tablet

A digitizer is a special table with a grid of fine wiring behind the face.

• You place your map on the digitize and then secure it with removable sticking tape.
• In your digitizing software, you then specify a coordinate system
• Then you digitize the bounding coordinates of the map.
• After the setup phase, you trace the map features using a magnetic pen (puck). The map features get sent to your computer as a GIS map.

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.

Digitizing can be very time-consuming and so its expensive.  However, once digitized, GIS maps can be used over-and-over. That’s one way digitizing becomes cost effective.

Digitizing can take time to learn and to master. It can be a real pain!

2. Heads up digitizing

A second manual digitizing technique is known as heads-up digitizing. To be honest, the other digitizing techniques in this post are geared towards industrial-scale digitizing. For example, whole-of-country map bases.

For small digitizing projects, heads-up digitizing is my preferred option. I use this GIS data input technique all the time in my GIS consultancy. It is where you…

1. Scan a map to create a digital version of it. For small maps, the flatbed scanner sitting on the desk next to you is good enough. You will need to scan bigger maps at an office centre.
2. 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.
3. On-screen digitize using the georectified map as a backdrop. 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 QGIS in my QGIS 3 georeferencing and digitizing course…

3. 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 rarely able 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.

Automatic scanning can also be very quick when the map being scanned is a single map theme eg. just roads or just watercourses (but not a map with 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.

4. 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 rarely used for natural resource applications in GIS and is mostly used for producing detailed cadastre maps.

5. 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.

GPS is also another means of data conversion.

Data Cleaning

The elephant in the room… Just because you have a map in a coordinate system, that does not mean its a GIS map. GIS maps need “clean” data. Linework with overshoots or undershoots, or polygons that are not closed are of little use in a GIS. So, while its important to have data input techniques, often the ones I’ve talked about here are only the beginning of the journey.

See also what is spatial data.

Conclusion

There are five main GIS data input techniques. 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.

Click this link to go to enrol in my 1.5 hour follow-along video QGIS 3 digitizing course

You might also be interested in this post about GIS data sources

References:

http://bgis.sanbi.org/gis-primer/page_25.htm

http://nerrs.noaa.gov/doc/siteprofile/acebasin/html/gis_data/gisint2.htm

http://www.amesremote.com/section4.htm

http://education.nationalgeographic.com/education/encyclopedia/geographic-information-system-gis/?ar_a=1