Spatial and Attribute Data

Spatial data (where)

• specifies location
• stored in a shape file, geodatabase or similar geographic file

Attribute (descriptive) data (what, how much, when)

• specifies characteristics at that location, natural or human created
• stored in a data base table

GIS traditionally maintain spatial and attribute data separately, then "join' them for display or analysis

• for example, in Mapview, the Attributes of...table is used to link a shapefile (spatial structure) with a data base table containing attribute information in order to display the attribute data spatially on map.

Representing Data with Raster and Vector Mode

Raster Model

• Area is covered by grid with (usually) equal - sized, square cells
• Attributes are recorded by assigning each cell a single value based on the majority featured (attribute) in the cell, such as land use type.
• Image data is a special case of raster data in which the "attribute" is a reflectance value from the geomagnetic spectrum

                           - cells in image data often called pixels (picture elements)

Vector Model

The fundamental concept of vector GIS is that all geographic features in the real work can be represented either as:

• points or dots (nodes): trees, poles, fire plugs, airports, cities
• lines (arcs): streams, streets, sewers
• areas (polygons): land parcels, cities, countries, forest, rock type

Because representation depends on shape, Mapview refers ro files containing vector as shapefiles

Projection, Scale, Accuracy and Resolution (the key properties of spatial data))

Projection: the method by which the curved 3-D surface of the earth is represented by X,Y coordinates on a 2-D flat map/screen

• distortion is inevitable

Scale: the ratio of distance on a map to the equivalent distance on the ground

• in theory GIS is scale independent but in practice there is an implicit range of scales for data output in any project

Accuracy: how well does the database info match  the real world

• Positional: how close are features to their real world location
• Consistency: do feature characteristics in database match those in real world

                               is  a road in the database a road in the real world?

• Completeness: are all real world instances of features present in the database?

                                 Are all roads included.

Resolution: the size of the smallest feature able to be recognized

                            for raster data, it is the pixel size

Basic of Map Projections

• A map projection is a mathematical model for conversion  of location from a three-dimensional earth surface to a two-dimensional map representation. This conversion necessarily distorts some aspect of the earth's surface, such as area, shape, distance, or direction.
• Projection make it possible to create maps of areas of the earth with as little distortion as possible. The projection process affects four properties: area, shape, distance and direction. There is no projection that maintains the integrity of all four properties at the same time.
• There are thousands of different map projections all depending on how they intersect earth's surface and how they are oriented. For example the line of latitude or longitude where a projection intersects or "cuts" the earth's surface is called the point of contact, or standard line, where distortion is minimized.

Common GIS Projections

• Mercator -  A conformal, cylindrical projection tangent to the equator. Originally created to display accurate compass bearings for sea travel. An additional feature of this projection is that all local shapes are accurate and clearly defined.
• Transverse Mercator - Similar to the Mercator except that the cylinder is tangent along a meridian instead of the equator. The result is a conformal projection that minimizes distortion along a north - south line, but does not maintain true directions.
• Universal Transverse Mercator(UTM) - Based on a Transverse Mercator projection centered in the middle of zones that are 6 degrees in longitude wide. These zones have been created throughout the world.
• Albers Equal Area Conic -  This conic projection uses two standard parallels to reduce some of the distortion of a projection with one standard parallel. Shape and linear scale distortion are minimized between standard parallels.

Coordinate System

• There are many different coordinate systems, based on a variety of geodetic datums, projections, and units use
• Geographic coordinate systems (no projection): Spheriod (or Ellipsoid) - based systems, local systems.
• Projected coordinate systems: world, continental, polar, US National Grids, UTM, state plane.

Coordinate System vs. Map Projections

• A map projection is a method  or a type of equation used to transform three-dimensional coordinates on the earth to two-dimensional coordinates on the map.
• A coordinate system usually includes the specification of a map projection, plus the three dimensional model of the Earth to be used, the distance units to be used on the map, and information about the relative positions of the two dimensional map and the model of the Earth.

ESRI

A geographic coordinate system is a coordinate system that enables every location on the Earth to be specified by a set of numbers. The coordinates are often chosen such that one of the numbers represent vertical position, and two or three of the numbers represent horizontal position. A common choice of coordinates is latitude, longitude and elevation.

Example of Coordinates System

2 Type of Coordinate System

• Geographic Coordinate System (GCS)
• Projected Coordinate System

WGS_1984_UTM_Zone_51N                                GCS_PRS_1992

Projection: Transverse_Mercator                              Datum:

False_Easting: 500000.000000                                 D_Philippine_Reference_System_1992

False_Northing: 0.000000

Cental_Meridian: 123.000000

Scale_Factor: 0.999600

Latitude_Of_Origin: 0.000000

Linear Unit: Meter

GCS_WGS_1984

Datum: D_WGS_1984

GIS Data Model

• Allows the geographic features in real world locations to be digitally represented and stored in a database so that they can be abstractly presented in map (analog) form, and can also  be worked with and manipulated to address some problem.