GisBlogger.com

Coordinate Reference System

The Coordinate Reference System can be divided into projected coordinate reference systems (also called Cartesian or rectangular coordinate reference systems) and geographic coordinate reference systems.

The Geographic Coordinate Reference System

The geographic coordinate system is one of the most common coordinate systems in use. It uses degrees of latitude and longitude to describe a location on the earth’s surface.

Lines of latitude run parallel to the equator and divide the earth into 180 equal portions from north to south (or south to north). The reference latitude is the equator and each hemisphere is divided into ninety equal portions, each representing one degree of latitude.In the northern hemisphere degrees of latitude are measured from zero at the equator to ninety at the north pole. In the southern hemisphere degrees of latitude are measured from zero at the equator to ninety degrees at the south pole. To simplify the digitization of maps, degrees of latitude in the southern hemisphere are often assigned negative values (0 to -90°). Wherever you are on the earth’s surface, the distance between lines of latitude is the same (60 nautical miles,), so they conform to the uniform grid criterion assigned to a useful grid system.

Lines of longitude, however, do not stand up so well to the standard
of uniformity. Lines of longitude run perpendicular to the equator and converge at the poles. The reference line for longitude (the prime meridian) runs from the North pole to the South pole through Greenwich, England. Subsequent lines of longitude are measured from zero to 180 degrees East or West of the prime meridian. Note that values West of the prime meridian are assigned negative values for use in digital mapping applications.

Only at the equator,does the distance represented by one line of
longitude equal to the distance represented by one degree of latitude. As you
move towards the poles, the distance between lines of longitude becomes
progressively less, until, at the exact location of the pole, all 360° of longitude
are represented by a single point that you could put your finger on (you
probably would want to wear gloves though). Using the geographic coordinate
system, we have a grid of lines dividing the earth into squares that cover
approximately 12363.365 square kilometers at the equator.

To be truly useful, a map grid must be divided into small enough sections so
that they can be used to describe the location of a point on the map within an acceptable level of accuracy. To accomplish this, degrees are divided into
minutes (') and seconds ("). There are sixty minutes in a degree, and sixty
seconds in a minute (3600 seconds in a degree). So, at the equator, one
second of latitude or longitude = 30.87624 meters.

 

Projected Coordinate Reference System

A projected coordinate system is a flat, two-dimensional representation of the Earth. It is based on a sphere or spheroid geographic coordinate system, but it uses linear units of measure for coordinates, so that calculations of distance and area are easily done in terms of those same units.

The latitude and longitude coordinates are converted to x, y coordinates on the flat projection. The x coordinate is usually the eastward direction of a point, and the y coordinate is usually the northward direction of a point.The center line that runs east and west is referred to as the x axis, and the center line that runs north and south is referred to as the y axis.

The intersection of the x and y axes is the origin and usually has a coordinate of (0,0). The values above the x axis are positive, and the values below the x axis are negative. The lines parallel to the x axis are equidistant from each other. The values to the right of the y axis are positive, and the values to the left of the y axis are negative. The lines parallel to the y axis are equidistant.

In a three-dimensional coordinate reference system, another axis, normally labeled Z, is added. It is also at right angles to the X and Y axes. The Z axis provides the third dimension of space.

Examples of commonly used projected coordinate reference system are the Universal Transverse Mercator(UTM) and  WGS84 commonly seen on most maps.

Labels: GIS, Tutorials

A map projection is any method used to portray all parts of a sphere or three-dimensional body(the round earth) on a plane or flat surface. A coordinate reference system(CRS) helps to define using coordinates how a  two dimensional projected map in a GIS is related to real places on the earth’s surface.Every flat map misrepresents the surface of the Earth in some way.Therefore the decision as to which map projection and coordinate reference system to use, depends on the regional extent of the area you want to work in, on the analysis you want to do and to a lesser extent on the availability of data. A basic knowledge of the properties of commonly used projections helps in selecting a map that comes closest to fulfilling a specific need.

Map Projections

Map projections are an important aspect of all mapping as all maps require the transformation of the surface of the spherical earth to a flat surface.       

The following are the importance of Projections to mapping and GIS:

• They are used to focus reader's attention
• Amplify and provide selective detail for map's message
• Most importantly,poorly chosen map projections can distort your thematic data

Families of Map Projections

The process of creating map projections can be visualized by positioning a light source inside a transparent globe on which opaque earth features are placed. Then projecting the feature outlines onto a two-dimensional flat piece of paper.The families of map projections depend on the kind of flattenable surface you are projecting the sphere onto.

Illustration 1: The three families of map projections. They can be represented
by a)planar projections , b) conical projections or c) cylindrical projections

 

Therefore there are three families of map projections. The azimuthal or planar,conical and cylindrical projections corresponding to projections onto a plane, a cone and a cylinder respectively.

Accuracy Of Map Projections

Map projection process always results in some kind of distortions. There are distortions of angular conformity, distance and area.

A map projection may combine several of these characteristics, or may be a compromise that distorts all the properties of area, distance and angular conformity, within some acceptable limit. Examples of compromise projections being the Winkel Tripel projection and the Robinson projection often used for world maps.

Map Projections with Angular Conformity 

Correct angular conformity can be maintained on a map. A map projection that retains this property of angular conformity is called a conformal or orthomorphic projection.They are usually used in situations where it is prudent to preserve angular conformity.Examples are the Lambert
Conformal Conic projection and the Mercator projection.It is useful in navigational or meteorological tasks.

Equal-Area Map Projections

A map projection in which areas on a sphere, and the areas of any features contained on it, are mapped to the plane in such a way that two are related by a constant scaling factor.Equal-area projections are also called equivalent, homolographic, homalographic or equiareal

Equal area projections result in distortions of angular conformity when dealing with large areas. Small areas will be far less prone to having their angles distorted when you use an equal area projection. Examples are  Alber's
equal area, Lambert's equal area and Mollweide Equal Area Cylindrical
projections.They are usually used in educational maps.

Equal Distance Map Projections

A map projection in which the distances between one or two points and every other point on the map differ from the corresponding distances on the sphere by only a constant scaling factor.A map is equidistant when it correctly represents distances from the center of the projection to any other place on the map. Equidistant projections maintain accurate distances from the center of the projection or along given lines.Usually used for radio and seismic mapping and navigation.Examples are Plate Carree Equidistant Cylindrical and Equirectangular projection.

 

I think this post is getting boring. I’ll continue later…………………….

Labels: GIS, Tutorials

 

Raster images come in the form of individual pixels. Each spatial location or resolution element has an associated pixel value, which indicates the coordinates, elevation, and any relevant attribute data, such as a color or ID number.

For GIS, CAD, or other mapping applications, raster image data is acquired by satellite or airborne sensors, such as GeoEye-1, Worldview-2, Worldview-1, QuickBird, or IKONOS, high resolution satellite sensors. The spatial resolution is determined by the resolution of the acquisition device, as well as the quality of the original data source. Because a raster image must have pixels for all spatial locations, the size of the represented spatial area is strictly limited. When the spatial resolution is doubled, the total size of a two-dimensional raster image increases by 400%, as the number of pixels is doubled in both X and Y dimensions. The same is true when a larger area is to be covered using the same spatial resolution.

Labels: GIS, Tutorials

A vector data provides a way to represent features in a GIS environment. A feature is anything which is visible on a landscape. A vector data can be defined as an abstraction of the real world where positional data is represented in the form of coordinates.In vector data, the basic units of spatial information are points, lines or polyline and polygon. 
A vector feature has its shape represented using geometry. The geometry is
made up of one or more interconnected vertices. A vertex describes a position in space using an x, y and optionally z axis. Geometries which have vertices with a z axis are often referred to as 2.5D since they describe height or depth at each vertex, but not both. A point consists of only a single vertex, Where the geometry consists of two or more vertices and the first and last vertex are not equal, a polyline feature is formed . Where four or more vertices are present, and the last vertex is equal to the first, an enclosed polygon feature is formed.Vector features have attributes, which consist of text or numerical information that describe the features.

Advantages: Data can be represented at its original resolution and form without generalization. Graphic output is usually more aesthetically pleasing (traditional cartographic representation); Since most data, e.g. hard copy maps, is in vector form no data conversion is required. Accurate geographic location of data is maintained. Allows for efficient encoding of topology, and as a result more efficient operations that require topological information, e.g. proximity, network analysis.

Disadvantages: The location of each vertex needs to be stored explicitly. For effective analysis, vector data must be converted into a topological structure. This is often processing intensive and usually requires extensive data cleaning. As well, topology is static, and any updating or editing of the vector data requires re-building of the topology. Algorithms for manipulative and analysis functions are complex and may be processing intensive. Often, this inherently limits the functionality for large data sets, e.g. a large number of features. Continuous data, such as elevation data, is not effectively represented in vector form. Usually substantial data generalization or interpolation is required for these data layers. Spatial analysis and filtering within polygons is impossible.

Labels: GIS, Tutorials

A geographic information system (GIS) is a computer-based tool for mapping and analyzing geospatial data.Geospatial data refers to information about the geographic location of an entity.This often involves the use of a geographic coordinate, like a latitude or longitude value.Spatial data,geographic data, GIS data, map data, location data, coordinate data and spatial geometry data are commonly used terms used to refer to geospatial data.

GIS technology integrates common database operations such
as query and statistical analysis with the unique visualization and geographic analysis benefits offered by maps. These abilities distinguish GIS from other information systems and make it valuable to a wide range of public and private enterprises for explaining events, predicting outcomes, and planning strategies.

Components of a Geographic Information System

A Geographic Information System seamlessly integrates five key components: hardware, software, data,people, and methods.

Hardware

Hardware includes the computer on which a GIS operates, the monitor on which results are displayed, and a printer for making hard copies of the results. Today, GIS software runs on a wide range of hardware types, from centralized computer servers to desktop computers used in stand-alone or networked configurations. The data files used in GIS are relatively large, so the computer must have a fast processing speed and a large hard drive capable of saving many files. Because a GIS outputs visual results, a large,
high-resolution monitor and a high-quality printer are recommended.

Software

GIS software provides the functions and tools needed to store, analyze, and display geographic information. Key software components include tools for the input and manipulation of geographic information, a database management system (DBMS), tools that support geographic query, analysis, and visualization, and a graphical user interface (GUI) for easy access to tools. The industry leader is ARCGIS, produced by Environmental Systems Research, Inc.

Data

Possibly the most important component of a GIS is the data. A GIS will integrate spatial data with other data resources and can even use a database management system,used by most organizations to organize and maintain their data, to manage spatial data.There are three ways to obtain the data to be used in a GIS. Geographic data and related tabular data can be collected in-house or produced by digitizing images from aerial photographs or published maps. Data can also be purchased from commercial data provider. Finally, data can be obtained from the federal government at no cost.

People

GIS users range from technical specialists who design and maintain the system to those who use it to help them perform their everyday work. The basic techniques of GIS are simple enough to master that even students in elementary schools are learning to use GIS. Because the technology is used in so many ways, experienced GIS users have a tremendous advantage in today’s job market.

Methods

A successful GIS operates according to a well-designed plan and business rules, which are the models and operating practices unique to each organization.

Labels: GIS, Tutorials