This article provides a brief overview of the GIS data collection process along with examples of methodologies for GIS data capture.
Pre-processing Data in GIS
When working on a GIS project, the first issue or decision that cartographers have to face is how to incorporate data into the system. This is the process called “data capture”. The fact that data capture is one of the most time consuming processes in GIS, turns the decision on how to collect data, one of the most important, since the cost of this part of the project may become a great burden for the rest of the future analysis that is going to be performed with this data.
There are different methodologies to capture data, but we could make a first big division, depending on the fact that we use preexisting data as the origin of our own data, or if we are going to create data basically from scratch. It is very important that the GIS analyst has a clear idea of what the project intends to analyze, because depending on the purpose of the study, one system or another could be the most appropriate at that moment.
|• Creating GIS Data
• Types of GIS Data
• Types of Error in GIS
• Digitizing Errors in GIS
• What is Metadata?
• GIS Glossary
Methodologies for GIS data capturing
This section reviews some of the most common ways GIS data is created. More detail about GIS data creation can be found in on this article: Methods for Creating Spatial Databases.
We can decide to digitize or scan data existing found on previously printed maps, resulting in vector or raster data as a base for further analysis. This methodology of data capturing may have the problem of how difficult is to get an up to date final product. If we use printed maps that are old, the data may be not current, and therefore, we are adding the error of out of data information to the whole analysis, affecting the result of the project. Also we have to consider the possible processing errors that digitizing can cause due to the mistakes made by the operator.
On the other hand, the decision on how to capture data may be taken by using survey methodologies. One of the advantages of this methodology is the possibility of enter the data directly in a digital format, with the consequent elimination of possibilities of error, and getting a more accurate product.
Among the survey methods of data capturing, we have to mention the Global Positioning Systems technology. Since GPS became a public tool, it has been one of the preferred tools to geolocate features for future mapping and analysis. The accuracy of GPSs has been improving year after year, getting also easier to use with each new generation of GPS units.
The technology known as COGO, or Coordinate Geometry, has become also a must in any GIS software that wants to offer a good service to its users. Land surveyors, Geologists, Civil Engineers, etc, have become daily users of this technology that gives you the flexibility of using geometry factors to delimitate features, such as curves, intersections, centerlines, etc.
Another methodology for data capture in GIS would be remotely sensed data. The different type of sensors attached to satellites, have made possible the affordability of imagery with an accuracy and resolution that was unthinkable only a couple of decades ago. The generalization of use of Remote Sensing data to Thematic mapping, was probably the biggest revolution in cartography since the invention of Compass.
The use of LiDAR technology (Light Detection And Ranging) is becoming more and more important as a source of GIS data. This technology uses a source of light, normally laser, to measure distances and angles. The flexibility of this tool, and the fact of using light as the main element, allows its use in multiple fields, like archaeology, geology, meteorology, etc.
Finally, there is another type of data capture that uses photogrammetric techniques. A big part of the digital data that cartographers use comes from photo interpretation. The old technique of stereo pairs that use to be done with stereoscopes, are now digitized and entered in computerized systems that allow to photo interpretation with a high level of accuracy.
Processing GIS data after capture
After capturing the data, but previous to its use in analysis, all data should be subject to a post processing phase that may or may not include editing and removal of capture errors. This process is known as QA/QC which stands for Quality Assurance / Quality Control. Again, this may be a very time consuming process, but the as costly as it may seem, it may actually save us time and money in further phases of our analytical process.
The final part of data capturing, will consist on providing the data with the right coordinate system and projection, possible format changes, (vectorial or raster) and any other process that our data requires in order to be effectively used for the type of analysis that we want to perform.
In general, we have to always remember that the three steps related to capturing data in a GIS project are equally important. The pre-processing will give us the right vision of what the analysis that we want to perform will require from the data that we need. The capture itself will bring, if correctly chosen and performed, the right level of accuracy and effectiveness desired to perform our analysis, and finally, the post-processing will eliminate errors and problems before the crucial moment of the analysis, when finding errors can become a too heavy burden to throughout the whole project.
Deciding the right steps in each one of these aforementioned phases will make our project more effective both from the analytical point of view and will improve the ratio quality-cost as well.