GIS Applications in Civil Engineering Note #3 Maps as Numbers GIS Data Structures Xudong Jia, Ph.D., P.E. January, 2011

Summary of the First Two Lectures How do we describe geographical features? by recognizing two types of data: Spatial data which describes location (where) Attribute data which specifies characteristics at that location (what, how much, and when) How do we represent these digitally in a GIS? by grouping into layers based on similar characteristics (e.g hydrography, elevation, water lines, sewer lines, grocery sales) and using either: vector data model (coverage in ARC/INFO, shapefile in ArcView) raster data model (GRID or Image in ARC/INFO & ArcView) by selecting appropriate data properties for each layer with respect to: projection, scale, accuracy, and resolution How do we incorporate into a computer application system? by using a relational Data Base Management System (DBMS)

Maps as Numbers Maps are represented by using numbers Binary and Decimal System. How to convert a binary value to a decimal number or vice versa. Hexadecimal number 00 to FF in one byte More examples on binary and decimal conversions.

Maps as Numbers Two data models for maps : Vector and Raster One data model for attributes: flat files Raster: It is a model that contains a grid. Each grid cell is a map unit used to represent a pixel. Cell size determines the resolution of the data. The grid has an extent. Each grid is owned by one feature Features cannot be perfectly fitted into cells. Cell’s bit depth provides a range to describe possible feature values.

Maps as Numbers Benefits of using Raster Model Easy to understand Capable of rapid retrieval and analysis Easy to draw on the screen and on computer devices which display pixels.

Maps as Numbers Vector Model Composed of points that are provided by exact coordinates Accuracy Efficient at storing features Able to store topological attributes for features.

Maps as Numbers Structuring Attributes A flat file as show on Page 84 of the Textbook Relational database table A distributed database through server extensions, database engines, and data warehouse. Data dictionary or metadata, the list of all the attributes along with all their characteristics save as a separte file or as a header of the file

Maps as Numbers Structuring Maps Vector Data Structures Cartographic Spaghetti

Maps as Numbers Structuring Maps Vector Data Structures Arc/Node Model

Maps as Numbers Structuring Maps Vector Data Structures Topological Arc/Node Model (see Page 88)

Maps as Numbers Structuring Maps Vector Data Structures Triangulated Irregular Network (TIN Model)

TIN: Triangulated Irregular Network Surface Maps as Numbers TIN: Triangulated Irregular Network Surface A B C D 6 1 2 3 4 5 E F G H Elevation points (nodes) chosen based on relief complexity, and then their 3-D location (x,y,z) determined. Points Elevation points connected to form a set of triangular polygons; these then represented in a vector structure. Polygons Attribute Info. Database Attribute data associated via relational DBMS (e.g. slope, aspect, soils, etc.) Advantages over raster: fewer points captures discontinuities (e.g ridges) slope and aspect easily recorded Disadvans.: Relating to other polygons for map overlay is compute intensive (many polygons)

Maps as Numbers Raster Data Structure Challenging issues on mixed pixel problem, redundant or missing data and large storage requirement

Maps as Numbers Full Matrix--162 bytes 111111122222222223 Run Length Encoding Full Matrix--162 bytes 111111122222222223 111111122222222233 111111122222222333 111111222222223333 111113333333333333 111333333333333333 1,7,2,17,3,18 1,7,2,16,3,18 1,7,2,15,3,18 1,6,2,14,3,18 1,5,3,18 1,3,3,18 Run Length (row)--44 bytes This is a “lossless” compression, as opposed to “lossy,” since the original data can be exactly reproduced. Now, GIS packages generally rely on commercial compression routines. Pkzip is the most common, general purpose routine. MrSid (from Lizard Technology)and ECW (from ER Mapper) are used for images. All these essentially use the same concept. Occasionally, data is still delivered to you in run-length compression, especially in remote sensing applications.

Maps as Numbers R-Tree Encoding See Page 92

Maps as Numbers Quad Tree Encoding See Page 92 Essentially involves compression applied to both row and column. sides of square grid divided evenly on a recursive basis length decreases by half # of areas increases fourfold area decreases by one fourth Resample by combining (e.g. average) the four cell values although storage increases if save all samples, can save processing costs if some operations don’t need high resolution for nominal or binary data can save storage by using maximum block representation all blocks with same value at any one level in tree can be stored as single value 3.25 3 4 3.5 2.5 2 4 5 3 4 2 4 4 4 4 1 4 2 4 3 2 store this quadrant as single 1 1 1 1 store this quadrant as single zero

Maps as Numbers Image Pyramid and Quad Encoding See Page 92

Maps as Numbers Why Topological Matters

Maps as Numbers Why Topological Matters

Maps as Numbers Why Topological Matters

Maps as Numbers Why Topological Matters

Maps as Numbers Why Topological Matters Terms: Slivers Spike Unsnapped Nodes Un-ended lines Why Topological Matters

Maps as Numbers Why Topological Matters Terms: Slivers Spike Unsnapped Nodes Un-ended lines Why Topological Matters

Maps as Numbers Formats for GIS Data Vector Data Formats: HPGL – Page Description Language PostScript - Page Description Language PDF – Portable Documen Format , GeoPDF AutoCAD DXF format DIME/TIGER Format DLG Format KML GML XML

Maps as Numbers Formats for GIS Data DIME/TIGER Data Model

Maps as Numbers Formats for GIS Data DIME/TIGER Data Model (Page 100)

Maps as Numbers Formats for GIS Data DLG Model

Maps as Numbers Raster Data Formats TIF GIF JPEF GEOTIP PNG Encapsulated PostScript DEM (1:24,000 and 1:100,000)

Maps as Numbers Exchange Data and Data Standards Data exchanges between Raster and Vector Data exchanges among different GIS platforms Data exchanges between different systsems SDTS (Spatial Data Transfer Standards) Open Geospatial Consortium Open GIS Specifications