2.1. Acquisition of Spatial and Attribute Data

 

Geographic data link location with time, and attributes. Location is essential in a geographic information system and are the basis for mapping and linking different kinds of information. Without locations, data are said to be "aspatial" and have no value at all within a geographic information system. In GIS, Time is an optional element. Many aspects of the earth's surface are slow to change and can be thought of as unchanging. Height above sea level changes slowly because of erosion and movements of the earth's crust, but these processes operate on scales of hundreds or thousands of years. On the other hand, atmospheric temperature changes daily, and dramatic changes sometimes occur in minutes. So, time is distinctly important. Attributes refer to descriptive information. The range of attributes in geographic information is vast. Some attributes are physical or environmental in nature (e.g., atmospheric temperature or elevation), while others are social or economic (e.g., population or income). There are five main types of attributes (also known as scales of measurement, explained in the next module): nominal, ordinal, interval, ratio, and cyclic.

The general types of GIS spatial data sources found today can be grouped into raster data sources and vector data sources.

Raster Data Sources

            Orthophotos and satellite images are two of the most referred to raster images. Both orthophotos and satellite data are called imagery but the orthophoto typically begins as a photographic product, which is scanned to become a digital image. The satellite image, on the other hand, is obtained electronically; there is no analog-to-digital conversion. All commercial satellite data today are wholly digital products.

Photograph-Based Imagery

            Earlier, the main source of raster data in GIS is a photograph-based imagery. This generally refers to an aerial photography aimed directly at the earth’s surface below the platform. A line projected through the camera lens to the ground is perpendicular to the earth’s surface. Such photographs are called vertical photographs. Photographs created when the angle of the line formed by the lens to the ground and the earth’s surface is not 90 degrees are referred to as oblique photographs. Although they can be scanned and make great images, they are not considered acceptable as GIS data.  Many scanned images are used in GIS both for illustrative and more serious, interpretive applications.

Ortho Photographs

Aerial photographs used in GIS need to have the distortion removed. In essence, an ortho photograph is a corrected, digitized aerial photograph. Ortho photographs combine the detail of photographs with the properties of a map. This means that distances and areas can be measured on the orthophoto, bearings can be taken, and the data can be combined with other GIS data. Orthophotos have been in existence a long time, although earlier orthophotos were primarily available only as analog products, such as a paper or film print. Image-processing advances have made it much easier to produce digital orthophotos that are stored as a computer image file.

Satellite Imagery

Satellite images form one of the most important sources of raster data for GIS and image processing software. As the name implies, imagery can be obtained from many earth-orbiting satellites providing data, gathered by a variety of sensors. Satellite data are referred to as imagery instead of photography because the vast majority of satellite data are collected through electronic sensors, and not by film based photographic process.

Digital Elevation Models

            Digital elevation models (DEMs) are one of the most versatile and useful GIS layers. These simple grid files contain locations (an x, y coordinate) with the elevation at that point (a z value) and support numerous topographical applications. The primary utility of these datasets comes from the fact that they represent a topographical surface (such as features on the earth) and show the interaction with physical landforms of all other data placed upon them. Other GIS datasets, both raster and vector, can be overlain on the DEM to provide a more realistic interaction between the data and the underlying topography. DEM data serve as the framework for many three-dimensional images and graphics.

Digital Raster Graphics

            Digital raster graphics (DRGs) are a relatively recent dataset created by scanning existing paper maps and creating an image with coordinates (so it can be georeferenced) suitable for combining with other GIS data. This has been most commonly applied to USGS quad maps, particularly the 7.5-minute products although many other paper maps could be scanned in the same manner. DRGs make excellent backdrops for vector data, providing additional context for the data you add. In fact, this capability is one the best characteristics of raster data. Be it a satellite image, orthophoto, digital elevation model, or DRG, raster backgrounds show how vector data interact with the surrounding region.

Satellite Data

            Digital data from earth-orbiting satellites provide a vast array of raster data suitable for use in a GIS. Data collected by variety of sensors in these satellites. Although there are several thousand man-made satellites around earth, only a few series currently provide data suitable for GIS. These satellites are referred to as earth-observation satellites in that they collect images (from a variety of sensors) of the earth’s surface and send these to receiving stations on the ground. There is one other important group of satellites that do not record imagery, but rather provide locational reference information concerning one’s location on the planet. These locations are frequently used in GIS to record locations of surface features and georeference GIS datasets. The global positioning satellites (GPS) are the most complete satellite series currently used for locational information.

Vector Data Sources

            Vector data model uses coordinates to store the shape of spatial objects. It uses the geometric objects of point, line, and polygon to represent spatial features. Besides using existing vector data, most GIS practitioners will end up making their own vector data. One can create vector data by digitizing off the computer screen (referred to as “heads-up” digitizing) or by digitizing a printed map on a digitizing board. The more common GIS vector datasets are described below.

Digital Line Graphs

            Digital line graphs (DLGs) are digital representations of the vectors found on USGS quadrangle map. A DLG consists of data pertaining to a particular theme. For example, all the elevation contours on the map represent one data theme. The same can be said for the water features, transportation features, manmade objects, and other feature groups. Each theme is considered to be a separate DLG.  To collect all the features, that we observe on the typical USGS quad digitally, we will need to get all the DLGs for that quad. DLGs are available in 1:24,000, 1:100,000, 1:250,000, and 1:1,000,000 scales.

SSURGO

            One recent addition to vector datasets has been the Soil Survey GIS (SSURGO) data of the U S Government. These datasets are digital versions of the common soil surveys read for many years in booklet form. The Natural Resource Conservation Service (NRCS, formerly known as the Soil Conservation Service) is a division of the U.S. Department of Agriculture and produces SSURGO data by counties. SSURGO data provide detailed soil information based on soil surveys and are updated when possible, from the most recent aerial photography and orthophotos.

Digital Chart of the World (DCW)

            One of the earlier universally used vector data sets is the small-scale (1:1,000,000) Digital Chart of the World, often abbreviated as DCW. The DCW is a basic digital dataset covering the entire planet and was derived from small-scale Operational Navigation Charts (ONC) The data contain common thematic layers, such as transportation, hypsography (elevation contours), hydrography, railroads, populated areas, and some land cover. The Operational Navigation Chart (ONC) are widely used for mission planning/analysis, intelligence briefings, and the preparation of visual cockpit navigational display. They provide a small-scale translation of the cultural and terrain features for the pilots/navigators flying at medium and low altitudes.

Collection of Attribute Data

            Attribute data are the description of the spatial data. They are the non-graphic information associated with a point, line, or area elements in a GIS. Attribute data are stored in tables and each row of a table represents a map feature. The sources of attribute data may be either existing datasets that are already compiled and available in some form or dataset generated by ourselves. In the second case while we may have complete control over the data gathering process, we generally have much more work to do.

 

Data types and their source in India

 

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