Coordinate systems
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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

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Coordinate Systems

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Coordinate systems

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

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

Types of Projections with examples

  • Planar (Azimuthal) - North Polar Stereographic

  • Conic – Lambert Conformal Conic, Albers Equal Area, Polyconic

  • Cylindrical – Mercator and Transverse Mercator


Coordinate systems

GIS is composed of layers

  • Layers

    • land/water

    • roads

    • urban areas

    • pollution levels

  • Data can be represented by VECTORS, or

  • Data can be represented by RASTERS


Coordinate systems

Vector GIS

  • Every object is given a location

  • Database consists of points, lines and polygons


Coordinate systems

Raster GIS

  • Every location given an object

  • Grid

    • Cells or pixels


Coordinate systems

Creating A Raster

  • Overlay grid on real world

  • give each cell a value


Coordinate systems

Cell Values

  • 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


Coordinate systems

Cell Values

  • Integer

  • Real Numbers

  • Alphabetic

  • Interpret using legend


Coordinate systems

Characteristics of a Raster

  • Resolution

    • smallest unit discernible

    • often grid size, but not always

  • Orientation

    • angle that grid makes with true north

  • Value

    • information stored in cell


Coordinate systems

Characteristics of a Raster

  • Zones

    • areas of continuous values

      • e.g. county, land parcels, etc.

  • Class - term used to describe all the zones of same value in a layer


Coordinate systems

Characteristics of a Raster

  • Location

    • each cell has a unique location

      • often defined by bottom left corner

      • X, Y coordinate


Coordinate systems

Storing the Information

  • Full length encoding

    • store every single cell’s value individually


Coordinate systems

Storing the Information

  • Run Length encoding

    • compress data


Coordinate systems

Storing the Information

  • Quadtree Encoding – compress data


Coordinate systems

Sources of Raster Data

  • Scanned Images

    • Aerial photographs

    • maps

  • Satellite images

  • Many packages work on RECTIFYING these images

    • photograph’s scale is not constant across image


Coordinate systems

Vector GIS

  • Every object is given a location

  • Database consists of points, lines and polygons


Coordinate systems

Creating a Vector GIS

  • 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.


Coordinate systems

Creating a Vector GIS

  • 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.


Coordinate systems

Creating a Vector GIS

  • Two Methods of Defining Polygons

    • Polygon Storage

    • Arc Storage

    • Used to represent lakes, landuse categories, forest stands, etc.


Coordinate systems

Polygon Storage

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

  • Each line encoded twice

  • Difficult to dissolve boundary


Coordinate systems

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

Topology

  • The geometric relationship between objects located in space.

    • Adjacency

    • Containment

    • Connectivity


Raster and vector summary

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.


Coordinate systems

  • 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


Coordinate systems

  • 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

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

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 formats1

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

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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