GIS RELATED FAQs

1. What is GIS?
2. What type of data can you view in a GIS?
3. What are the two types of spatial data?
4. What is map projection?
5. What does NAD27 and NAD83 mean?

 

Q: What is GIS?

A: Geographic Information Systems (GIS) store information about the world as a collection of thematic layers and display this information as maps. Layers include digital basemaps and datasets such as elevation, roads, streams, rock types, and structural data, and a variety of other geospatial datasets. GIS is the tool that allows you to see relationships between them.
 

Q: What type of data can you view in a GIS?

A: The two types of data you can view using GIS software are graphic and attribute data. Graphic data are spatial elements organized into homogeneous layers. DEMs, topographic maps, satellite images, and digital orthophotography are all examples of graphic data. Attribute data are non-graphic elements that contain information about spatial elements. Lithology type, contact type, structural measurement type are all examples of attribute data. The acquisition, manipulation, and maintenance of data are the most expensive components of a GIS.
 

Q: What are the two types of spatial data?

A: Geographic information systems work with two fundamentally different types of spatial data: the vector model and the raster model. Vector data is well suited for describing discreet features, such as summits or well points, while raster data is well suited for displaying values that change over a larger area, such as elevation data or vegetation. Vector data models information about points, lines, and polygons as a x,y coordinates. The location of a point feature, such as a summit, can be described by a single x,y coordinate. Linear features, such as highways or political boundaries are a series of point coordinates. Polygon features, such as floodplains or lake boundaries are stored as a closed loop of coordinates.

Raster data models continuously varying features such as soil type, vegetation, or elevation. A raster image is a collection of grid cells where each cell has a value. The most common example of GIS raster data are hard copy maps or digital orthoimagery. The raster data are stored in a .TIF format. Each .TIF image is georeferenced (tied to the ground) using text based World Files (.TFW)

Q: What is map projection?

A: Map projection is "a mathematical model used to transform positions on the surface of the earth, which is curved, onto a flat map surface." Map projection is most relevant when combining spatial data from different sources. If the projections don’t match, neither will the data.

Q: What does NAD27 and NAD83 mean?

A: NAD refers to the North American Datum and is a set of parameters that helps define a spherical coordinate system. There are two main datums used in the US–NAD27 and NAD83. The center point for NAD27 is Meades Ranch in Kansas and used the Clarke spheroid of 1866. The center point for NAD83 is the center of the earth's mass and uses the GRS80 spheroid which factors in the earth's equatorial bulge.

Q: What is a Digital Orthophoto Quadrangle (DOQ)?

A: A Digital Orthophoto Quadrangle (DOQ) is a digital, uniform-scale image created from aerial photos. It is a photographic map in which ground features are displayed in their true ground position, because relief displacements caused by the camera and terrain of an aerial photograph have been removed. It combines the image characteristics of a photograph with the geometric qualities of a map, thus it is possible to get direct measurements of distances, areas, angles, and positions from a DOQ.

A DOQ can be used on-screen to collect, review, and revise other digital data, especially digital line graphs (DLG) and topographic maps. When the DOQ is combined with other digital products, such as digital raster graphics (DRG) or digital elevation models (DEM), the resulting image provides additional visual information for the extraction and revision of base cartographic information. (From USGS factsheet)

Q: What are the differences between a DOQ and an aerial photograph?

A: An aerial photograph and an orthophoto or orthoimage may look alike but there are several important differences that allow an orthophoto to be used like a map. A conventional perspective aerial photograph contains image distortions caused by the tilting of the camera and terrain relief (topography). It does not have a uniform scale. You cannot measure distances on an aerial photograph like you can on a map. An aerial photo is not a map. The effects of tilt and relief are removed from the aerial photograph by a mathematical process called rectification. An orthophoto is a uniform-scale image. Since an orthophoto has a uniform scale, it is possible to measure directly on it like other maps. An orthophoto may serve as a base map onto which other map information may be overlaid.(From USGS factsheet)

Q: What is a Digital Line Graph (DLG) ?

A: A Digital Line Graph (DLG) is digital vector data representing cartographic information. DLGs contain a wide variety of information depicting geographic features (for example, hypsography, hydrography, boundaries, roads, utility lines, etc). DLGs are derived from hypsographic data (contour lines) using USGS 7.5-minute, 15-minute, 2-arc-second (30- by 60-minute), and 1:2 million-scale topographic quadrangle maps. (From USGS factsheet)

Q:What is a Digital Raster Graphic (DRG)?

A: A digital raster graphic (DRG) is a scanned image of a U.S. Geological Survey (USGS) topographic map. The scanned image includes all map collar information. The image inside the map neatline is georeferenced to the surface of the Earth. The DRG can be used to collect, review, and revise other digital data, especially digital line graphs (DLG). When the DRG is combined with other digital products, such as digital orthophoto quadrangles (DOQ) or digital elevation models (DEM), the resulting image provides additional visual detail for the extraction and revision of base cartographic information. (From USGS Factsheet)

Q:What is a Digital Elevation Model (DEM) ?

A: A Digital Elevation Model (DEM) is digital cartographic/geographic data in raster form. The terrain elevations for ground positions are sampled at regularly spaced horizontal intervals. DEMs are derived from hypsographic data (contour lines) and/or photogrammetric methods using USGS 7.5-minute, 15-minute, 2-arc-second (30- by 60-minute), and 1-degree (1:250,000-scale) topographic quadrangle maps. (From USGS factsheet)

Q:What is GeoTIFF ?

A: GeoTIFF format is a non-proprietary geographic TIFF format. The purpose
of GeoTIFF is to provide information that lets raster imagery (scanned
maps, satellite images, results of geographic analysis, etc) be read
automatically into correct position and scale within many GIS
softwares. GeoTIFF implements a tag structure which embeds the
geographic information methodically and interoperably (and invisibly to
most users) inside the TIFF file.

Q:What is the Digital Chart of the World (DCW)?

A: The Digital Chart of the World is a 1.7 GB digital geographic database
that is available on CD-ROM. It was input from 1:1,000,000 Operational
Navigation Charts and 1:2,000,000 Joint Navigation Charts. It includes
17 layers, aeronautical info, data quality info, drainage, supplemental
drainage, hypsography, hypsography supplemental, land cover, ocean
features, physiography, political/ocean, populated places, railroads,
transportation structure, utilities, and vegetation.

Q:What is SDTS?

A: SDTS is the acronym for the Spatial Data Transfer Standard. SDTS is a FIPS(173) and FGDC (STD-002) standard that defines a non-proprietary format for packaging vector or raster spatial data with attributes, metadata, a data quality report and usually a data dictionary. SDTS is primarily intended to be used for spatial data product distribution and archiving. The format and structure of a SDTS file set is designed to enable blind transfer of information between different hardware/software environments without loss of contextual information.

Q:Where can I get SDTS information on the Internet?

A: The U.S. Geological Survey's SDTS web site is at:
http://mcmcweb.er.usgs.gov/sdts

Documents and sample data sets are at:
ftp://sdts.er.usgs.gov/pub/sdts

Q:What is a Landsat image?

A: The purpose of the Landsat program is to provide the world's scientists and application engineers with a continuing stream of remote sensing data for monitoring and managing the Earth's resources. Landsat 7 is the latest NASA satellite in a series that has produced an uninterrupted multispectral record of the Earth's land surface since 1972. The Landsat-7 system is designed to collect 7 bands or channels of reflected energy and one channel of emitted energy. These eight bands of data are used to discriminate between Earth surface materials through the development of spectral signatures. For any given material, the amount of emitted and reflected radiation varies by wavelength. These variations are used to establish the signature reflectance fingerprint for that material.

The ETM+ ground samples at three different resolutions; 30 meters for bands 1-5, and 7, 60 meters for band 6, and 15 meters for band 8. In comparison, IKONOS, the recently launched Space Imaging sensor, has an IFOV (or pixel size, which in this case, is the area of terrain or ocean covered by the field of view of a single detector) of 1 meter. The French SPOT panchromatic sensor an IFOV of 10 meters whereas the SPOT multispectral (XS) sensor has an IFOV of 20 meters. (from NASA)