Data Acquisition

 

Geographic information system integrates spatial data with attribute data. Spatial data refers to the data that describes the location of objects and phenomenons. Attribute data on the other hand refers to the description of the objects and phenomenons.

Acquisition of spatial and attribute data can be classified into two; Primary data acquisition and Secondary data acquisition.

Primary Data Capture

Primary data capture is a method in which data is collected by means of direct field observations. In the case of vector data, directly captured data commonly comes from two sources. They are Global Navigational Satellite System (GNSS) and total station. Total stations are specialized, primary data capture instruments that measures horizontal and vertical angles, with a tool to measure the slope distance. It helps us to quickly and accurately derive the topography for a particular landscape.

GNSS consists of a network of navigation satellites situated around the globe that provides precise coordinate information for any point on the earth’s surface. These locations can be collected as individual points or can be linked together to form lines or polygons depending on user preference. Attribute data such as land-use type, telephone pole number, and river name can be simultaneously entered by the user. This location and attribute data can then be uploaded to the GIS for visualization.

GPS is becoming increasingly incorporated into mobile phones now a days. This facilitate real-time data capture and sharing to the masses. The wide spread availability of this technology led to a development of crowdsourced data acquisition initiatives. Crowdsourcing is a data collection method whereby users contribute freely to building spatial databases. This rapidly expanding methodology is utilized in applications such as Google Earth and Bing Maps.

Raster data obtained via direct capture comes more commonly from remotely sensed sources. Remotely sensed data offers the advantage of collecting data without any physical contact with the object being imaged. In addition, huge tracts of land can be covered with little to no additional time and labour. On the other hand, validation is required for remotely sensed data to ensure that the sensor is not only operating correctly but properly calibrated to collect the desired information. Satellites and aerial cameras are the important sources of direct-capture raster data.

Secondary Data Capture

Secondary data capture is an indirect methodology that utilizes the vast amount of existing geospatial data available in both digital and hard-copy formats. Secondary data are available from a variety of sources including international agencies, federal governments, state governments, university websites and commercial websites. Often these data are free of cost.

Vast quantity of secondary sources of data are non-digital in character. In order to use in GIS, these data has to be converted into a digital format. Three primary methods exist for digitizing spatial information: two are manual, and one is automated.

Tablet digitizing or Heads down digitizing

In this method, a digitizing tablet and a digitizing puck is used to trace the points, lines and polygons of a hard-copy map into a computer. The digitizing puck is similar to a multi-button mouse with a cross hair. The resulting digital file has to be properly georeferenced to ensure that it will properly align with existing datasets. It is a useful technique because of its ability to accurately copy maps in poor condition. It is more time consuming than Heads-up digitizing.

Heads-up digitizing or On screen digitizing

This method involves scanning a map or image into a computer. The digitizer then traces the points, lines and polygons using digitizing software. This method of digitizing has been named "heads-up" digitizing because the focus of the user is up on the screen. It is also known as “on-screen” digitizing. It replaced Manual digitizing because of its speed and accuracy. It is, however, limited to using scans of high quality maps and images.

The automated method of secondary data capture requires the user to scan a paper map and vectorize the information therein. This vectorization method typically requires a specific software package that can convert a raster scan to vector lines. This requires a very high-resolution, clean scanned imagery. If the image is not clean, all the imperfections on the map will likely be converted to false points/lines/polygons in the digital version. This method is much quicker than the manual digitizing methods and may be the best option if time is a limiting factor. It is often followed by a heads-up digitizing session to edit any errors that occurred during automation.

The final secondary data capture method worth noting is the use of information from reports and documents. Through this method, one enters information from reports and documents into the attribute table of an existing, digital GIS file. For example, new information specific to census tracts may become available following a scientific study. The GIS user simply needs to download the existing GIS file of census tracts and begin entering the study’s report/document information directly into the attribute table. If the data tables are available digitally, the use of the “join” and “relate” functions in a GIS are often extremely helpful as they will automate much of the data entry effort.


Popular posts from this blog

How to Georeference in QGIS?

     One of the fundamental and preliminary work that every GIS enthusiast should learn first is georeferencing. It is one of the elementary steps in most GIS.   It is the process of taking an airphoto, or a scanned topographic map, and adding geographic information to the image so that GIS software can 'place' the image in its appropriate real world location. This process is completed by selecting pixels in the digital image and assigning them geographic coordinates.  Usually, these coordinates are obtained by doing proper field surveys and collecting coordinates with a GPS device for few easily identifiable features in the image or the map.   More frequently, an image is georeferenced to an existing image that already has embedded geographic information.   Georeferencing raster data allows it to be viewed, queried, and analyzed with other geographic data.   Georeferencing in QGIS       One of the great benefits of QGIS is the...
Read more

2.4. Raster Data

  The raster data model represents the Earth’s surface by means of an array of two-dimensional grid cells called pixels. Each pixel has an associated value, which represents the characteristics of the spatial phenomenon. These cells or pixels are used as building blocks for creating points, lines, areas, networks, and surfaces. Raster data represent points by single cells, line by sequences of neighbouring cells, and areas by collection of contiguous cells. The JPEG, BMP, and TIFF file formats are based on the raster data model. All liquid crystal display (LCD) computer monitors are based on raster technology as they are composed of a set number of rows and columns of pixels. Although pixels may be triangles, hexagons, or even octagons, square pixels are most common. Raster models are useful for storing data that varies continuously, such as an aerial photograph, a satellite image, a surface of chemical concentrations, or an elevation surface.  Elements of Raster Data Model...
Read more

2. Modelling the Real World - Data Types and Models - Vector and Raster

Image
  The Earth’s surface is really complex, and hence, it is exceedingly difficult to represent the real-world features in a Geographical Information System. So, we use to simplify the complex real-world features before we input them into GIS. This simplified features that that we input into a GIS is called an entity.             Real world features are of two basic forms: Objects and Phenomena. Objects are discrete, identifiable units that have some sort of spatial reference, usually in the form of geographic coordinates. They have well defined boundaries. For example, houses, factories, roads, rivers, or lakes. Phenomena are distributed continuously over a large area. For E.g. Terrain, temperature, rainfall. In GIS, the real world is represented in these two basic forms. Thus, there are two distinct approaches of representing the real world in geographic databases. These two models that help to represent complex earth’s surfac...
Read more