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## PowerPoint Slideshow about ' Coordinate Systems' - mariel

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Coordinate Systems

- Global Coordinate System – Latitude, Longitude and elevation
- UTM – eastings and northings, reference points are the equator and the central meridians (60 of them for 60 zones) Extends from 84° N to 80 °S
- State Plane – State specific, northings and eastings NAD 27 is based on feet, NAD 83 is based on meters.
- There are many many more.

Reference Ellipsoids and Datums

- Clark 1866 spheroid – NAD 27
(Sometimes Clark 1880?)

- GRS 80 spheroid – NAD 83
- WGS 84 spheriod – NAD 83
- Sphere – for world projections
- Many many more.

Types of Projections with examples

- Planar (Azimuthal) - North Polar Stereographic
- Conic – Lambert Conformal Conic, Albers Equal Area, Polyconic
- Cylindrical – Mercator and Transverse Mercator

- Layers
- land/water
- roads
- urban areas
- pollution levels

- Data can be represented by VECTORS, or
- Data can be represented by RASTERS

- Every object is given a location
- Database consists of points, lines and polygons

- Every location given an object
- Grid
- Cells or pixels

- Overlay grid on real world
- give each cell a value

- Assume only one value per cell in any given layer
- How do you decide what value to give the cell?
- Value of greatest proportion?
- Value of the most important feature?
- Some systems allow for percent composition, edge effect

- Integer
- Real Numbers
- Alphabetic
- Interpret using legend

- Resolution
- smallest unit discernible
- often grid size, but not always

- Orientation
- angle that grid makes with true north

- Value
- information stored in cell

- Zones
- areas of continuous values
- e.g. county, land parcels, etc.

- areas of continuous values
- Class - term used to describe all the zones of same value in a layer

- Location
- each cell has a unique location
- often defined by bottom left corner
- X, Y coordinate

- each cell has a unique location

- Full length encoding
- store every single cell’s value individually

- Run Length encoding
- compress data

- Quadtree Encoding – compress data

- Scanned Images
- Aerial photographs
- maps

- Satellite images
- Many packages work on RECTIFYING these images
- photograph’s scale is not constant across image

- Every object is given a location
- Database consists of points, lines and polygons

- Define Points
- fundamental property of a vector GIS
- no dimensions, but they have a location
- can have attributes associated with it
- used for utility poles, sampling points, wells etc.

- Define Arcs (lines)
- line that joins points
- also called chains and edges
- has length and direction
- attributes can include what is on right and left side
- used to represent road, utility lines, rivers, etc.

- Two Methods of Defining Polygons
- Polygon Storage
- Arc Storage
- Used to represent lakes, landuse categories, forest stands, etc.

- Store Polygon as series of points, starting and ending at same point

- Each line encoded twice
- Difficult to dissolve boundary

Polygon Storage - Use Arcs(more common)

- Every arc stored as a sequence of points
- Every polygon stored as series of arcs
- Boundaries only stored once
- Nodes are points where arcs meet or end

Topology

- The geometric relationship between objects located in space.
- Adjacency
- Containment
- Connectivity

Raster and Vector Summary

- Vectors have advantage of accuracy but not good with continuous fields
- Vectors were used first - digitizing
- Earliest include ASCII (x,y coordinates but got too large) then binary took over.
- Raster not good with lines or points but good with continuous coverage areas.
- Raster has the mixed pixel problem.

- Raster Advantages:
- Simple data structure
- Compatible with remotely sensed or scanned data
- Simple spatial analysis procedures

- Raster Disadvantages:
- Requires greater storage space on computer
- Depending on pixel size, graphical output may be less pleasing
- Projection transformations are more difficult
- More difficult to represent topological relationships

- Vector Advantages
- Requires less disk storage space
- Topological relationships are readily maintained
- Graphical output more closely resembles hand-drawn maps

- Vector Disadvantages
- More complex data structure
- Not as compatible with remotely sensed data
- Software and hardware are often more expensive
- Some spatial analysis procedures may be more difficult
- Overlaying multiple vector maps is often time consuming

Maps as numbers

- Binary 0000 1111
- Eight bits in a row are termed a byte
- 256 conbinations or 0 to 255

- ASCII text- American Standard Code for Information Interchange - 256 standard meanings for the values that fall into one byte. (letters, numbers, special characters)

Vector Data Formats

- DXF Digital Exchange Format (Autocad) ASCIIw/binary code mantains layers
- ArcView Shapefiles 9 (.shp)
- HPGL Hewlett-Packard Graphics Language
- A device-specific but industry standard language for defining vector graphics in page coordinates (ASCII) - no topology

- Adobe PostScript ASCII

Vector Data Formats

- DLG Digital Line Graph - USGS (1:100,000 & 1:24,000) ASCII UTM
- TIGER U.S. Census Bureau
(Topologically Integrated Geographic Encoding and Referencing)

- Geocoded block address-matching
- Topology correct, but accuracy problems

Raster Data Formats

- DRG Digital Raster Graph
- TIF Tagged Interchange Format
- GIF Graphics Interchange Format
- JPEG Joint Photographic Experts Group
- GeoTiff – Geographical referenced TIF Files

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