GIS Midterm Study Guide.txt
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What is GIS
- Geographic- Where is it? X,Y
- Information- What is it?
- System- How if this stuff converted, communicated, analyzed, stored,
- Science- Method for explaining observations about the world.
GIS is different because it includes ____?
- Adjacency- a region can have neighbors
- Linkage- a line can connect to others
- Inclusion- a point can be inside a region
- Proximity- a line can be near a point
What are the two kinds of GIS?
- Vector (points lines and polygons)
- Raster (cells in a grid)
Vector GIS represents what?
- Points- discrete locations, too small to be depicted as areas, stored as a single pair of X,Y coordinates
- Lines- represent objects that have length but are too narrow to be depicted as areas
- Polygons- series of connected line segments forming an enclosed area, represent objects too large to be depicted as points or lines.
What is a Shapefile?
Shapefiles are ArcGIS’s native file format for geographic features and associated attribute data.
- Stores geographic data
- 3 components feature classes, feature dataset, and non-spatial tables.
Large vs Small Scale:
- Large scale- small area (zoomed in) smaller the second digit in the ratio the larger the scale
- Small scale- large area ( zoomed out)
- Small number, large scale, small area
What is a projection?
A 2-D representation of a 3-D object.
What is compromised when you project a 3-d object?
Area, Shape, Distance, Direction.
What shapes can be unrolled to lie flat?
Cylinder, Cone and Plane
- Tangent found by placing paper around outside of globe, the point where the paper touches the globe is the tangent.
- Secants are found by putting a tube of paper that intersects the earth.
- A light source that is parallel to the earth’s axis makes the latitude lines spread out as you approach the equator.
- If the light source is in the center of the globe the latitude spacing gets closer as you approach the equator.
- Conic projections are created by setting a cone over a globe and projecting light from the center of the globe onto the cone.
- If a cone passes through the globe, it intersects along two lines secants
- Longitude lines are projected onto the conical surface, where they meet at the apex. Latitude lines are projected onto the cone as concentric rings.
- Used for most polar maps. In its most basic form, a plane touches the globe at a single point, which is known as the point of tangency. There is zero distortion at this point.
- Planar projections tend to have a large amount of distortion at the edges and zero distortion at the point of tangency
- Equatorial- parallel to the Earths rotation axis
- Transverse- at 90 degrees to the Earth’s rotation axis
- Oblique- at any other angle
Common map projections:
- Conformal- preserves shapes and angles
- Equal Area- preserves area
- Equidistant- preserves the distance from one or two specified points to all other points on the map
- Azimuthal- All directions are true from a single specified point ( usually the center ) to all other points on the map.
- Compromise- no point is completely distortion free; distortion is minimized near the center along the equator.
When do you perform a reprojection?
- Take into consideration what the purpose is of your map as to what you are willing to compromise in the projection and what needs to be preserved.
- When the data is not in the same format as the projection you want to use you must reproject the data. This prevents misalignment issues when you have datasets that use different geographic coordinate systems.
- What projection should I use?
- If shape is important, use a conformal projection
- If areas are important, use an equivalent projection.
- If distance is important, use equidistant.
Metadata- metadata is information that describes, or documents, a geographic dataset
- Location, reference
- Latitude and Longitude ex: UTM (meters), State Plane (feet)
- Assignes where we are after you’ve projected a map, Coordinate systems are units of measure used to locate a position within a specific projection. Standardized method for assigning codes to locations so that locations can be found using the codes alone. Standardized coordinate systems use absolute locations.
Geographic Coordinate System:
- Measures angular distances, degrees, minutes, seconds. Latitude, longitude,
- UTM, Stateplanes
- The earth is a spheroid- not actually a perfect sphere so it is a geoid. Different approximations of the geoid result in datum.
- NAD83, WGS84
Share information, Highlight patterns and processes, illustrate results
Principles that enable goals to be achieved:
- Clarity- how message is portrayed
- Order & balance- logical organized elements of the map
- Hierarchy-intellectual importance
- Aesthetics/ Harmony- pleasing to the reader
- Contrast- distinguish between features
Important considerations to effective design:
- Purpose- of the map, goals and objectives for the design, what is the desired outcome
- Audience- know them, who are they? What do they know?
- Topic- What is the subject matter?
- Feedback loops- before, during and after
- Figure and ground, Legend, Orientation (N arrow), Scale, Title and Subtitle, Border/Neatlines, Insets, Who (mapmaker), When (date, revised), Source of data, coordinate systems, datum, explanatory notes.
- Nominal- not ordered ex five different genotypes
- Ordinal- one matters more than the other but the difference between the numbers doesn’t actually mean anything.
- Interval- difference between two values is meaningful like difference between temperatures.
- Ratio- similar to an interval but has a clear definition of what 0 is. Like temp 0 doesn’t mean no temperature.
Classification of features:
- Equal interval- divides the range of attributes values into equal-sized sub-ranges. Then the features are classified based on those ranges.
- Defined Interval- classifies polygon features by finding breakpoints so that the total area of a polygon in each class is approximately the same.
- Quantile- each class contains the same numbers of features
- Natural Breaks- between classes using a statistical formula (default classification with Jenks optimization), minimizes the sum of the variance within each of the classes.
- Standard Deviation- Finds the mean value and then places class breaks above and below the mean intervals
What is a table?
Contains attribute information, rows and columns
Not all spatial data contains attribute tables, sometimes attribute tables need to be joined to the spatial data in order for it to be accessible in a GIS
- A way to request information
- Attribute queries- Select by attribute
- Spatial queries- Select by location
3 Classes of Operators:
- Comparison- =, >, <
- Spatial- intersect, contain, within a distance of
- Logical- like, in, not
Why would you use select by attribute or select by location tools instead of a geoprocessing tool.
There are many ways to make spatial selections in ArcGIS. Use of the Select by Attribute and Select by Location are perhaps better than the Clip geoprocessing tool since the former methods preserves the complete polygon whereas the Clip saves only the area that intersects with the polygon layer you are using to clip with (the buffer, in our case).
How do you describe a grid?
Number of rows, number of columns, cell size, xy coordinates of the origin, no data value.
How do you describe a cell?
- Cell size- specificed in map units of feet or meters
- Cell area- square size of the cell size
- Rows X Columns=number of cells in the grid
- Number of cells X cell area= spatial extent.
What is a tradeoff of using raster instead of vector?
Storage requirements are large but the processing time is short.
Discrete Data in Raster Form:
Representation of distinct spatial objects that belong to a class, they have known and definable boundaries
What gets gridded in a raster?
You can convert a point line or polygon into a raster form.
Raster vs vector:
- Raster: better for field data, used my most imaging systems, can be compressed, easy to display and analyze, Forestry, Remote Sensing, Terrain Analysis, Hydrology
- Vector : Precise, Can represent precise coordinates, assumes feature model of landscape. Good for land parcels, census data, precise positional data, networks.
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