Scale
Ratio of map unts to GIS units
Vector Data Model
- Use discrete elements such as points, lines, and polygons to represent the geometry of real world entities3. Features are stored as a series of x-y coordinate system in geometry types: points, lines, or polygons
- Each feature has attributes or info
- Feature class is a collection of similar objects with the same attributes
Rastor Data Model
- Uses grid cells arranged in rows and columns with a value assigned to each grid• Uniform, regular cells
• Each cell, attributes are constant
Precision of raster model
• Half the cell size is how precise you can be
• Finer grid size=more precise also larger file (more data)
Spaghetti Vector Model
- Each line is stored separately in the database
- Lines and polygons can overlap
- Shared polygon boundaries are stored twice
- Since lines stored twice, storage is inefficient
Topological Vector Model
- Shares lines (state of Illinois will share with Missouri, used for parcel maps, very powerful)
- Complex data set up
- Geometric properties that do not change when bent or stretched
- Bending polygon doesn’t change neighbor because they bend also
- Planar topology (2D, no overlapping lines)
- Exhaustive (no gaps in polygon)
- Three tables
- Polygon (contains all polygons)
- Node (all nodes)
- Link (begin and end of nodes)
- Cons (cost more, clean data entry)
- Pros (Analysis, efficiency)
Types of Vector Files
ESRI Coverage
Shapefile
Feature Class/Geo Data Base
ESRI Coverage
• Kind of a pain, cease up when there are spaces in saving
• Three basic relations
o Connectivity (arcs connect at nodes)
o Area definition (are is series of connected arcs)
o Contiguity (arcs have left and right direction)
• Really old school
• Lines stored once
• Pro (land boundaries, move one line and other follows is critical)
• More complex than shapefiles due to topology
• Important to move files carefully so don’t miss key component (e00)
• They concentrate on storing topology. So you will see that the emphasis is in storing the geometry elements first, that is the nodes, edges that make up all the geometries. You will then see a separate set of tables that relate those geometries to the attributes
• Nodes at every node intersection
• Not have two or more nodes on top of eachother
• Have sense of direction
• Work well for topological relations (parcel boundaries)
• Doesn’t work well for 3D like a bridge
Shapefile
Spaghetti vector (lines stored twice)
• .shp file
• Pro (smaller file, quick retrieval, thematic)
• Is an open specification that was simple to implement
• Has a large library
• Each geometry worried about itself and not other geometries that it intersected
• No complicated math to insure correct
• Can have multiple geometries cross, and two points on top of eachother
Feature Class/Geo Data Base
Newest one
• Supports geographic concepts
• Contains multiple points, polygon, and polyline layers
Large Scale vs. Small Scale
- Only dealt with it in the annoying preset layers, preset zoom on the scale
- GIS terminology, large and small are reversed
- Map units/real units….1:24000 is a small fraction hence small scale, a 1:2 scale will be considered a large scale
Four types of data Quality
Geometric Accuracy
Thematic Accuracy
Temporal
Precision
Geometric Accuracy
Does it reflect actual geometry of the map (is it shifted?)
Thematic Accuracy
Did we pick the right them for the map
Temporal
When was data created? (shorline is good example)
Precision
How precise measuring depends how precise the layer was made, example length of rods, depends •
Metadata
- State has really strict codes
- Has notes with the map to help show how accurate, who made it, how it was made, etc.
- ArcCatalog has these notes
Geo Databases
Way arc prefers to store stuff, shape file has the .shp on the end of file, also looks like cylinder
Map Document vs. Layers
- Separation of info allows reality to be seen as a series of overlapping layers of info
- Each layer can be treated separately, but also combined
- Ex. Soils, floodplains, zoning, topography
Moving Files in ArcMap
• Understand the shape files aren’t just one file and there are 4-5 pieces attached with it
Types of Coordinate systems
Unprojected
Projected
Geoid/Ellipsoids
Unprojected
• Based on spherical globe coordinates, degrees of lat and long
Projected
- Converts spherical coordinates to planar using math equations
- Projects 3D to 2D map
- Every projection based on a GCS, ever GCS has datum, so every projection has a datum
Geoid/Ellipsoids
• Geoid is the lumpy shape that the earth actually is
• Projections are all about choosing an ellipsoid that represents the shape of the earth
• You can make an ellipsoid a sphere, but usually have one longer that represents earth a little better
• Defined with major and minor axis
• Geoid-earth is lumpy not perfect ellipse (theoretical surface defined by gravity measurements)
1. Complex so usually use ellipse (another source of error)
Datum
- Major and Minor access of the ellipsoid and where it is centered at on earth
- Minimize error from geoid and ellipse use a datum (shifts ellipse relative to geoid)
- Local and geocentric/world datum
- Word we say for ellipsoid
- It is both the technical specs of the ellipsoid and survey benchmarks
- Named after the names they are done for (North American)
- Must know and record datum to use data correctly later
