DOC PREVIEW
UNC-Chapel Hill GEOG 370 - Raster Data Model

This preview shows page 1-2-15-16-17-32-33 out of 33 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 33 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 33 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 33 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 33 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 33 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 33 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 33 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 33 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

Raster Data ModelSpatial Data ModelsA few synonyms for rasterSlide 4Raster data exampleSlide 6Slide 7Cells - Absolute ValuesCells - Coded ValuesGrids and missing dataCell Size & ResolutionRaster Data Model - ObjectsRaster Data Model - PointsRaster Data Model - LinesRaster Data Model - AreasRaster and Vector Data Model ComparisonRules for Assigning Cell ValuesRaster Data Model - StorageRaster Data Model – CompactionRaster Data Storage – No CompactionRaster Data Storage – Run Length EncodingRaster Data Compression Models: Block EncodingRaster Data Compression Models: Raster Chain CodesRaster Data Compression Models: QuadtreesRaster Data Compression Models: Quadtrees ExampleVector to Raster TransformationsVector Data Model - AdvantagesVector Data Model - DisadvantagesRaster Data Model - AdvantagesRaster Data Model - DisadvantagesWhich Data Model Should You Use?Common Examples of Raster DataSlide 33Raster Data ModelSpatial Data Models•Rasteruses individual cells in a matrix, or grid, format to represent real world entities•Vector uses coordinates to store the shape of spatial data objectsA few synonyms for raster•Surface•GRID – the ArcInfo raster type•Image (generic) – usually relates to satellite imagery•Image (.img) – Raster data structure used by Erdas Imagine, a common image-processing software package•Array – more technical term associated with how raster data are managed by computer programmers•Matrix – rarely used because of it’s association with mathematics, but it does occasionally come upRaster Data Model•In the raster data model, the primary data object is the cell or pixel•You are familiar with these if you have used a digital camera or viewed a computer monitorRaster data example75 70 79 80 7812 50 80 81 8015 14 79 78 6910 12 9 85 8010 11 10 80 90Cell (x,y)•The raster data model represents the Earth’s surface as an array of two-dimensional grid cells, with each cell having an associated value:1 2 3 5 84 6 8 3 93 5 3 3 17 5 4 3 92 2 4 5 2Cell valueCell size = resolutioncolumnsrowsRaster Data ModelRaster Data Model•Every cell has a value, even if it is a special value to indicate that there is “no data” or that data is “missing” at that location•The values are numbers, either:–actual values OR–codes representing an attributeCells - Absolute Values•In this instance, the value of the cell represents the value of the phenomenon of interest, e.g. the elevation at that pixel location.Cells - Coded Values•Here, the values stored in each cell are used as substitutes for categorical data, e.g. land cover classes:Grids and missing dataClarke, K.C., Figure 3.8: GIS data layer as a grid with a large section of “missing data,” in this case, the zeros in the ocean off of New York and New JerseyCell Size & Resolution•The size of the cells in the raster data model determines the resolution at which features can be represented•The resolution can have an effect on which features are represented in what locations:10 m Resolution1 m Resolution5 m ResolutionThe raster data model still represents spatial objects, but does so differently from the vector model:Geographic Primitives•Points–0 dimensional •Lines–1 dimensional•Polygons–2 dimensionalRaster Data Model - Objects++++1 point = 1 cellWhat problems do we have here? • 2 points in single pixel• Point on the boundary between 2 or more cellsRaster Data Model - PointsA line = a series of connected cells that portray length Problems with this representation? • Lines may be narrower than pixels show• Curved lines can loose detail (e.g., if the curves are smaller than the pixel resolution can detect)Raster Data Model - LinesArea = a group of connected cells that portray a shapeWhat problems could we have with this representation?• What if a lake’s edge falls in a pixel? • Area calculations loose accuracy Raster Data Model - AreasReal World Features Raster Vector • • • “A raster model tells what occurs everywhere, while a vector model tells where every thing occurs”Raster and Vector Data Model ComparisonRules for Assigning Cell Values•Cell values can be assigned to cells accorded to some set of rules, and selecting those rules differently can also effect the representation of features:Raster Data Model - Storage•There is a trade-off between spatial resolution and data storage when we use the raster data model, e.g.–60 km satellite image with 10m cell size•6000 X 6000 = 36,000,000 cells•1 byte of attribute value (i.e. values 0-255)•~36 MB of disk storage!–60 km satellite image with 100m cell size•600 x 600 = 360,000 cells•360 KB of data… 1% the size of the other oneRaster Data Model – Compaction•Because the raster data model records a value for each and every cell in a grid, it is very storage intensive, meaning that it can use a lot of memory and disk space to represent a theme •Compaction techniques are used in conjunction with raster data to reduce the amount of required storage space to a more manageable amount10, 10, 10 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 00 0 0 0 1 1 1 1 0 00 0 0 0 1 1 1 1 0 00 0 1 1 1 1 1 1 0 00 0 1 1 1 1 1 1 0 00 0 1 1 1 1 1 1 0 00 0 1 1 1 1 1 1 0 00 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0max. cell valuerowscolumnsProblem: too much redundancy1110010000100000000000000001110011100111000000000000000000000011100111001110000000000111001110011100Raster Data Storage – No CompactionThis approach represents each cell individually in the file:103 values10,10,10, 100, 100, 4, 1, 4, 0,20, 4, 1, 4, 0,20, 2, 1, 6, 0,20, 2, 1, 6, 0,20, 2, 1, 6, 0,20, 2, 1, 6, 0,20, 100, 10There is a tendency towards spatial autocorrelation; for nearby cells to have similar values - values often occur in runs across several cells 1110010000100000000000000001110011100111000000000000000000000011100111001110000000000111001110011100Raster Data Storage – Run Length EncodingThis approach takes advantage of patterns in the data, taking advantage of the repetition of values in a row:45 valuesrow by rowheaderRaster Data Compression Models:Block EncodingFrom An Introduction to Geographic Information Systems, Heywood et al. (2002)Run-length encoding in 2-D: Uses a series of square blocks to encode dataRaster Data Compression Models:Raster Chain CodesFrom An Introduction to Geographic Information Systems, Heywood et al. (2002)Reduces data by defining the boundary of entityRaster


View Full Document

UNC-Chapel Hill GEOG 370 - Raster Data Model

Documents in this Course
Lab 4

Lab 4

5 pages

Lab 1

Lab 1

3 pages

Lab 1

Lab 1

3 pages

Lab 3

Lab 3

3 pages

Lab 4

Lab 4

4 pages

Load more
Download Raster Data Model
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Raster Data Model and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Raster Data Model 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?