GIS QUESTIONS AND ANSWERS

1. What is projection? A Projection is a series of transformations which convert the location of points on a curved surface (the reference surface or datum) to locations on flat plane (i.e. transforms coordinates from one coordinate reference system to another). A map projection is one of many methods used to represent the 3-dimensional surface of the earth or other round body on a 2-dimensional plane in cartography (mapmaking).

2. What are different projection systems? Azimuthal projections Azimuthal projections touch the earth to a plane at one tangent point; angles from that tangent point are preserved, and distances from that point are computed by a function independent of the angle. Distance from the tangent point on the map is equal to surface distance on the earth. Azimuthal equal-area projection: distance from the tangent point on the map is equal to straight-line distance through the earth. Azimuthal conformal projection is the same as stereographic projection.

Conformal projections Conformal map projections preserve angles. Mercator projection wraps a cylinder around the earth; the distance from the equator on the map is being geographical latitude, on a scale where the earth’s radius is 1. Stereographic projection touches a plane to the earth and projects each point in a straight line from the antipode of the tangent. Equal-area projections These projections preserve area.

3. What is a coordinate system? Coordinate systems enable geographic datasets to use common locations for integration. A coordinate system is a reference system used to represent the locations of geographic features, imagery, and observations, such as Global Positioning System (GPS) locations, within a common geographic framework. Each coordinate system is defined by the following: Its measurement framework, which is either geographic (in which spherical coordinates are measured from the earth's center) or planimetric (in which the earth's coordinates are projected onto a two- dimensional planar surface) Units of measurement (typically feet or meters for projected coordinate systems or decimal degrees for latitude- longitude)

The definition of the map projection for projected coordinate systems Other measurement system properties such as a spheroid of reference, a datum, one or more standard parallels, a central meridian, and possible shifts in the x- and y- directions

4. Mention Types of coordinate systems The following are two common types of coordinate systems used in a geographic information system (GIS): A global or spherical coordinate system such as latitude- longitude. These are often referred to as geographic coordinate systems. A projected coordinate system such as universal transverse Mercator (UTM), Albers Equal Area, or Robinson, all of which (along with numerous other map projection models) provide various mechanisms to project maps of the earth's spherical surface onto a two- dimensional Cartesian coordinate plane. Projected coordinate systems are referred to as map projections. Coordinate systems (both geographic and projected) provide a framework for defining real-world locations.

5. What is a spatial reference? A spatial reference is a series of parameters that define the coordinate system and other spatial properties for each dataset in the geodatabase. It is typical that all datasets for the same area (and in the same geodatabase) use a common spatial reference definition. A spatial reference includes the following: The coordinate system The coordinate precision with which coordinates are stored (often referred to as the coordinate resolution) Processing tolerances (such as the cluster tolerance) The spatial extent covered by the dataset (often referred to as the spatial domain)

6. What is Geographic coordinate systems A geographic coordinate system (GCS) uses a three- dimensional spherical surface to define locations on the earth. A GCS is often incorrectly called a datum, but a datum is only one part of a GCS. A GCS includes an angular unit of measure, a prime meridian, and a datum (based on a spheroid). The spheroid defines the size and shape of the earth model, while the datum connects the spheroid to the earth's surface. A point is referenced by its longitude and latitude values. Longitude and latitude are angles measured from the earth's center to a point on the earth's surface. The angles often are measured in degrees (or

in grads). The following illustration shows the world as a globe with longitude and latitude values: In the spherical system, horizontal lines, or east–west lines, are lines of equal latitude, or parallels. Vertical lines, or north–south lines, are lines of equal longitude, or meridians. These lines encompass the globe and form a gridded network called a graticule.

The line of latitude midway between the poles is called the equator The line of latitude midway between the poles is called the equator. It defines the line of zero latitude. The line of zero longitude is called the prime meridian. For most GCSs, the prime meridian is the longitude that passes through Greenwich, England. The origin of the graticule (0,0) is defined by where the equator and prime meridian intersect. Latitude and longitude values are traditionally measured either in decimal degrees or in degrees, minutes, and seconds (DMS). Latitude values are measured relative to the equator and range from –90° at the south pole to +90° at the north pole. Longitude values are measured relative to the prime meridian. They range from –180° when traveling west to 180°

when traveling east. If the prime meridian is at the Greenwich, then Australia, which is south of equator and east of Greenwich, has positive longitude values and negative latitude values.

7. What is Geographic (datum) transformations If two datasets are not referenced to the same geographic coordinate system, you may need to perform a geographic (datum) transformation. This is a well-defined mathematical method to convert coordinates between two geographic coordinate systems. As with the coordinate systems, there are several hundred predefined geographic transformations that you can access. It is very important to correctly use a geographic transformation if it is required. When neglected, coordinates can be in the wrong location by up to a few hundred meters. Sometimes no transformation exists, or you have to use a third GCS like the World Geodetic System 1984 (WGS84) and combine two transformations.

8. What is Projected coordinate systems A projected coordinate system (PCS) is defined on a flat, two- dimensional surface. Unlike a GCS, a PCS has constant lengths, angles, and areas across the two dimensions. A PCS is always based on a GCS that is based on a sphere or spheroid. In addition to the GCS, a PCS includes a map projection, a set of projection parameters that customize the map projection for a particular location, and a linear unit of measure.

9. Compare Geographic vs. Projected Coordinates Advantages of the spherical coordinate system—You can represent any point on the Earth’s surface as accurately as your measurement techniques allow. The system itself does not introduce errors. Disadvantages of a spherical coordinate system—You will encounter complex and time-consuming arithmetic calculations in determining the distance between polygon longitude two points or the area surrounded by a by a set of points. Latitude- determined numbers plotted directly on paper in a Cartesian coordinate system result sometimes greatly distorted—figures. in distorted—

Advantages of a projected coordinate system on the Cartesian plane—Calculations of distances between points are trivial. Calculations of areas are relatively easy. Graphic representations are realistic, provided the area covered is not too large. Disadvantages of a projected coordinate system on the Cartesian plane—Almost every point is in the wrong place, although maybe not by much. All the projections introduce errors. Depending on projection, these errors are in distances, sizes, shapes, or directions.

10. What is a scale.? The scale of a map is the ratio of a distance on the map to the corresponding distance on the ground. Representation of scale Map scales may be expressed in words (a lexical scale), as a ratio, or as a fraction. Examples are: 'one centimetre to one hundred metres' or 1:10,000 or 1/10,000

11. What is geocoding? Geocoding is a process is the process of assigning locations to addresses to that they can be placed as points on a map, similar to putting pins on a paper map, and analyzed with other spatial coordinates to geocoding. data. The process the original data, assigns geographic hence the name 12. What is reverse geocoding? Reverse geocoding is the process of back (reverse) coding of a point location (latitude, longitude) to a readable address or place name. This permits the identification of nearby street addresses, places, and/or areal subdivisions such as neighborhoods, county, state, or country.

13. What is geo-referencing? Georeferencing is the process of taking a raster image or vector coverage, assigning it a coordinate system and coordinates, and translating, transforming, and warping/rubbersheeting it into position relative to some other spatial data, such as survey locations, street intersections, etc. This can be sometimes also be called rectification or georectification interchangeably, while in some contexts, georeferencing is considered to only include the assigning of a spatial reference and coordinates to the image, and rectification is the transformation and resampling of the image to remove distortion (as in orthorectification).

14. What is geo-processing? Geoprocessing is a GIS operation used to manipulate spatial data. A typical geoprocessing operation takes an input dataset, performs an operation on that dataset, and returns the result of the operation as an output dataset. Common geoprocessing operations include geographic feature overlay, feature selection and analysis, topology processing, raster processing, and data conversion. Geoprocessing allows for definition, management, and analysis of information used to form decisions. 15. What is BLOB? A Binary Large OBject (BLOB) is a collection of binary data stored as a single entity in a database management system. Blobs are typically images, audio or other multimedia objects, though sometimes binary executable code is stored as a blob. Database support for blobs is not universal.

16. What is the full meaning of GPS? The Global Positioning System (GPS) is a space- based satellite navigation system that provides location and time information, anywhere on or near the earth where there is an unobstructed line of sight to four or more GPS satellites.

17. The role of GPS in GIS. The uses of GIS, GPS, and RS technologies, either individually or in combination, span a broad range of applications and degrees of complexity. Simple applications might involve determining the location of sampling sites, plotting maps for use in the field, or examining the distribution of soil types in relation to yields and productivity. More complex applications take advantage of the analytical capabilities of GIS and RS software. These might include vegetation classification for predicting crop yield or environmental impacts, modeling of surface water drainage patterns, or tracking animal migration patterns. Precision Agriculture Forest Management Habitat Analysis Data Analysis and Display

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