Data Collection & Creation

Coordinate Geometry (COGO)

Coordinate geometry is a keyboard-based method of spatial data entry. This method is most commonly used to enter cadastral or land record data. This method is highly precise as entering the actual survey measurements of the property lines creates the database. Distances and bearings are entered into the GIS from the original surveyor plats. The GIS software builds the digital vector file from these values.

Geocoding

Geocoding is another keyboard-based method. Geocoding uses addresses from a flat file (such as a .dbf file, MS Access database or excel spreadsheet) to create x,y coordinate locations interpolated from a geocodable spatial database. These spatial databases are most commonly street centerline files but can be other types. The resultant geocoded database is a point file. The geocode process can be done in most of the GIS softwares, such as ArcGIS, QGIS, Ilwis. It can also be done online using Google API and ArcGIS Online. In the learning resources you will find some tutorials on how to execute this procedure.

Global Positioning Systems (GPS)

GPS is a highly accurate navigation system using signals from satellites to determine a location on the Earth’s surface, irrespective of weather conditions. Originally devised in the 1970s by the Department of Defense for military purposes. Through interpolation, these satellite signals received by a data logger can pinpoint the holder’s location. Depending on the unit, the locational accuracy can reach to the millimeter. Combined with attribute data entered at the time of collection, GPS is a rapid and acccurate method of data collection. For more information access Trimble’s excellent tutorial on GPS.

Surveying and mapping was one of the first commercial adaptations of GPS, as it provides a latitude and longitude position directly without the need to measure angles and distances between points.

However, it hasn’t entirely replaced surveying field instruments such as the theodolite, Electronic Distance Meter, or the more modern Total Station, due to the cost of the technology and the need for GPS to be able to ‘see’ the satellites therefore restricting its use near trees and tall buildings.

In practice, GPS technology is often incorporated into a Total Station to produce complete survey data. GPS receivers used for base line measurements are generally more complex and expensive than those in common use, requiring a high quality antenna.

There are three methods of GPS measurement that are utilised by surveyors.

• Static GPS Baseline. Static GPS is used for determining accurate coordinates for survey points by simultaneously recording GPS observations over a known and unknown survey point for at least 20 minutes. The data is then processed in the office to provide coordinates with an accuracy of better than 5mm depending on the duration of the observations and satellite availability at the time of the measurements.
• Real Time Kinematic (RTK) Observations. This is where one receiver remains in one position over a known point – the Base Station – and another receiver moves between positions – the Rover Station. The position of the Rover can be computed and stored within a few seconds, using a radio link to provide a coordinate correction. This method gives similar accuracy to baseline measurements within 10km of the base station.
• Continuously Operating Reference Stations (CORS). This where a survey quality GPS receiver is permanently installed in a location as a starting point for any GPS measurements in the district. Common users of CORS are mining sites, major engineering projects and local governments. Surveyors’ GPS receivers can then collect field data and combine it with the CORS data to calculate positions. Many countries have a CORS network that are used by many industries. Australia’s CORS network is the Australian Regional GPS Network, and uses an online processing system to deliver data over the internet within 24 hours, and give positions within an accuracy of a few centimetres. Local CORS networks are also used to provide instant positions similar to the RTK method by using a mobile phone data link to provide a coordinate correction to the surveyor and their rover.

Learn more about the equipment available at the GIS Center to survey your data.

Image Processing

Geodatasets can be derived from digital imagery. Most commonly satellite imagery is utilized in a process called supervised classification in which a user selected a sampling of pixels for which the user knows the type (vegetation species, land use, etc). Using a classification algorithm, remote sensing software such as R, ERDAS, ENVI classifies a digital image into these named categories based on the sample pixels. In contrast to the other methods discussed, supervised classification results in a raster dataset.

LIdar

LIdar is for Light Detection And Ranging is a remote sensing product that measures distance to a target using pulsed laser. Differences in laser return times and wavelengths can be then used to make 3D models and DEM and DSM. In common basis, Lidar has been used in airborne to scan the surface. Some new technology allows to use ground laser to get information on a perspective of the objects in the surface.

GIS data collection (download)

You can find GIS data available in many different sources and websites to download. One thing that is important in this process of getting the metadata. The metadata is the file that will tell more about the data, how it was created, the source, date, author, coordinate system. With this metadata you will be able to say if this data will fit in your needs in terms of scale, error, method. be derived from digital imagery. In this session you will find some links where you can search and download spatial data.