TUTORIAL ARC GIS GETTING TO KNOW ARCGIS DESKTOP
Chapter 6: Manipulate data georeferenced
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| Tutorial ArcGIS: Manipulate data georeferenced part1 |
It is, therefore, necessary to reference the data in precisely locating the surface of the earth (that is called georeferencing). If the entities are not exactly positioned or if forms used to represent objects are not correct, use a map or in GIS analysis of uniting spatial relationships do not give appropriate results.
It is therefore important to understand and determine their coordinates.
What georeferencing?
Georeferencing is the process of establishing a relationship between the data displayed in your Geographic Information System and their position in the real world.
Georeferencing is the process of establishing a relationship between the data displayed in your Geographic Information System and their position in the real world.
The earth is round, and the cards are flat, the conversion of geographical positions of a surface curve on a flat surface requires the use of a mathematical formula called projection Mapping.
To use the data in your GIS, it is necessary to reference their position on the surface of Earth.
Using a coordinate system, georeferencing transform spatial data geospatial data.
Georeferencing includes three different types of operations:
the transformation of spatial data Geospatial by the declaration of the coordinate system, transforming a system geographical coordinates to another, converting the spatial data processing Algebraic (translation, rotation ...).
what is a card?
- A map is a flat representation, simplified surface land.
- Any object (or phenomenon) is locatable in space capable of being represented geographically on a map.
A map is a planar geometric representation, simplified and conventional of all or a part of the Earth's surface, in a similar report, agreed: the scale.
Any object or phenomenon localized on the surface of the globe is likely to be mapped.
Historic
The first cards were designed to be able to identify and heading into space.
They represented fixed geographic objects to locate the great elements of space and occupation (relief, during the water, roads ...).
The evolution of the spatial representation of needs and the changing character of the parameters of human geography has resulted in a change of objective cartographic document.The first cards were designed to be able to identify and heading into space.
They represented fixed geographic objects to locate the great elements of space and occupation (relief, during the water, roads ...).
Besides providing location information and retrieval of large sets of fixed space, the card provides spatial visualization phenomena in many areas and allows the detection of correlations.
It remains one of the best tools for communication of information in many areas.
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A system of map coordinates to assign your spatial data to a location corresponds to the location of objects on the surface of the Earth.
To locate or measure features on a two-dimensional surface, a coordinate system Planar is more appropriate.
locate
- Geographic coordinates system
- Planar coordinate system
A system of map coordinates to assign your spatial data to a location corresponds to the location of objects on the surface of the Earth.
To locate or measure features on a two-dimensional surface, a coordinate system Planar is more appropriate.
Geographic coordinate system
The most commonly used reference system for locating objects on the surface of the Earth is a spherical coordinate system measured in latitude-longitude.
The most commonly used reference system for locating objects on the surface of the Earth is a spherical coordinate system measured in latitude-longitude.
This system can be used to position a point anywhere on the surface of the Globe.
Because of its ability to localize points, the spherical coordinate system is commonly called the Geographic Coordinate System or Global Reference System yet.
Longitudes and latitudes are angles measured from the center of the Earth and anywhere placed on the surface of the earth.
Longitude measure an angle east or west while a measured latitude angle north or south.
Longitudes lines, also called meridians, extend from the North Pole to the South Pole.
The latitude lines also called parallels, encircle the globe parallel rings.
Latitude and longitude are traditionally measured in degrees, minutes and seconds or degrees decimal.
Latitude and longitude are traditionally measured in degrees, minutes and seconds or degrees decimal.
Longitudes values vary from 0 ° to the meridian (Greenwich) to 180 ° when one moves to the east and from 0 to -180 ° when one moves westward.
Latitudes values vary 0 ° (Ecuador) at 90 ° when it moves toward the north, and from 0 ° to -90 ° when moving toward the south.
Coordinate systems planar
Because of the difficulties encountered when taking distances in a coordinate system spherical, the geographical data are projected in a planar coordinate system.
On one planar surface, the positions of the points are identified by x and y coordinates of a grid whose origin is the center of the grid.
Each point is located by a pair of coordinates by the reference to the center of the grid.
The first coordinate marks the horizontal position of the point and the second the vertical position of this point.
These two values are called X and Y coordinates
A map projection is a method for converting the 3-dimensional surface of the Land in a cartographic 2D flat surface. It may represent the entire surface land or a portion according to our needs.
Map Projection
- The conversion of a geographical positions curved surface to a flat surface
Principle
The principle is quite simple. Early cartographers imagined representing an area the planar projection of the objects placed on the surface of the Earth from a lamp.
The principle is quite simple. Early cartographers imagined representing an area the planar projection of the objects placed on the surface of the Earth from a lamp.
This light source can be placed at the center of the earth, on the opposite side of the globe relative to the planar surface, or a distance from the Earth.
The two-dimensional surface can be a cylinder, a plan or a cone.
Today
Today, all projections use mathematical formulas to convert data of a system of geographic location (latitude-longitude) to a flat surface.
Today, all projections use mathematical formulas to convert data of a system of geographic location (latitude-longitude) to a flat surface.
Although mathematical functions are used to calculate the planar coordinates of the different objects on the surface of the Earth, the term projection is still used to describe the set of rules control existing distortions on different maps.
Difficulties
Projecting a curved surface are two problems difficult to manage:
• Determination of the shape and dimensions of the Earth, it is necessary to model the surface
project.
• the projection on either side of this form in the map: one must choose the projection algorithm.
The concept of distortion
A map projection can bring the geographic point of a curved surface to a flat surface.
Projecting a curved surface are two problems difficult to manage:
• Determination of the shape and dimensions of the Earth, it is necessary to model the surface
project.
• the projection on either side of this form in the map: one must choose the projection algorithm.
Large projection families
The concept of distortion
A map projection can bring the geographic point of a curved surface to a flat surface.
This flattening process causes alterations in shape, the surface, the distance, and direction.
Fortunately, one can choose from several projection systems according to their ability to represent particular portions of the earth's surface and to preserve the distance, the surface or management.
Some projections minimize distortion of a particular property at the expense of others, while other projections generally minimize alterations.
We distinguish different projection, large families:
Conformal projection
These projections preserve the angles on a plane observed on the Earth's surface but change the area ratios. They are the most used. Lambert projections (I, II, III, IV and 93) used for representing the French territory are part of this family.
Conformal projection
These projections preserve the angles on a plane observed on the Earth's surface but change the area ratios. They are the most used. Lambert projections (I, II, III, IV and 93) used for representing the French territory are part of this family.
Equivalent projections
This type of projection maintains the area ratios and change angles. They are much used in statistics and earth sciences.
This type of projection maintains the area ratios and change angles. They are much used in statistics and earth sciences.
These projections include projection Mollweide.
The other families of projections
There are also other projection systems used much less:
There are also other projection systems used much less:
the equidistant projections which preserve distances interrupted projections...
In France, the most used are the projections Lambert projections. The cones projections Lambert is tangent to the ellipsoid along a contact parallel also called parallel origin (from latitude L0).
The top of the cone is located on the line of the poles. The developed cone on a plane gives
Lambert projection.
The origin of the projection corresponds to the intersection between the central meridian forming the Y-axis (Paris meridian) and the parallel origin forming the X-axis coordinates of this origin was modified (for example 600 km by 200 km + X and Y for the Lambert I, II and III) so as to have any negative coordinate onStore projection information
Associate projection files
According to the different data formats, it is possible to store this information in a part of the game of data. In most cases, this information must be stored in a text file associated with the data set.
According to the different data formats, it is possible to store this information in a part of the game of data. In most cases, this information must be stored in a text file associated with the data set.
For shapefiles and coverages, this file is called file projection (.prj file extension) while in the case of image or CAD file, we talk about file georeferencing.
The geodatabase is unique in the sense that it stores itself Information spatial reference in the database and thus eliminates the need to use a separate file.
Viewing projection information
The projection information associated with features in the layer is available from the application ArcCatalog ArcGIS via the Fields tab of the Layer Properties dialog box.
When the user selects the Shape field, all the spatial properties of the layer appears.
The type of entities stored in the layer is available (although not modifiable) and the reference space wherein data is stored.
The type of entities stored in the layer is available (although not modifiable) and the reference space wherein data is stored.
To view all the parameters associated with this coordinate system, just click on the gray button to the right of the name of the system reference. The buttons in the lower part of the dialog box to a user experienced possibly modify the parameters
Inform projection information
Inform projection information
If projection information has not been defined in advance by the user, ArcCatalog shows Unknow spatial reference as the value (unknown). By clicking on the gray button to the right of this value ArcCatalog opens a dialog box named spatial reference properties with all the tools of the definition of the projection system.
By clicking the selection button, ArcCatalog offers the user to select from himself all predefined coordinate systems, one that corresponds to the source of these data.
The Apply button allows ArcCatalog to automatically create the projection file or georeferencing corresponding to the data. When this file is created, the projection information is stored with the data source and any application of ArcGIS can exploit.
The Apply button allows ArcCatalog to automatically create the projection file or georeferencing corresponding to the data. When this file is created, the projection information is stored with the data source and any application of ArcGIS can exploit.
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