Introduction to Cartographic Modeling RNR 419/519 Introduction to Cartographic Modeling

Analysis vs. Synthesis Analysis – the separating of an entity its constituent elements or the manipulation and assessment of data to better understand or represent its form and function. RNR 473/573 – Fall Synthesis – the combining of constituent elements into single and unified entity, typically done through a model. RNR 419/519 - Spring

Definitions An analysis method is spatial if the results depend on the locations of the objects being analyzed Move the objects and the results change Results are not invariant under relocation Spatial concepts (location, distance, adjacency) appear explicitly Spatial analysis requires both attributes and locations of objects a GIS has been designed to store both

Spatial Modeling A model is used to integrate data to create information. A model is a simplification (logical or physical) of a process or system. Elements (thematic layers) represent inputs into the model that are integrated, usually mathematically, to create the output, the new information. In Cartographic Modeling the output is typically a new thematic layer

Spatial Processes: General Principles Integration of data through the GIS: * Linking data through common location–the thematic layer cake * Linking processes across disciplines–spatially explicit processes–e.g. social-economic and biophysical processes interact at common locations * Place-based modeling – representation of a location

GIS Database A GIS database consists of a set of thematic layers representing different entity types. Data may include: Vector Data Raster Data Images CAD Draws Associated Tables Graphics and Text http://www.ncddc.noaa.gov/technology/gis

Rasters and Vectors How to represent phenomena conceived as fields or discrete objects? Two basic data structures. Raster Divide the world into square cells, rectangle matrix Register the corners to the Earth Represent discrete objects as collections of one or more cells Represent fields by assigning attribute values to cells More commonly used to represent fields than discrete objects

Rasters Most rasters are uniform tessellation of square cells, but raster elements do not need to be square, nor must a raster be uniform.

Continuous Information Each cell has a value that represents the average elevation for the cell.

Discrete Types Legend Mixed conifer Douglas fir Oak savannah Grassland Raster representation. Each color represents a different value of a nominal- scale field denoting land cover class.

Characteristics of Rasters Pixel size The size of the cell or picture element, defining the level of spatial detail All variation within pixels is lost Assignment scheme The value of a cell may be an average over the cell, or a total within the cell, or the commonest value in the cell It may also be the value found at the cell’s central point

Effect of a raster representation using: the largest share rule (B) the central point rule

Vector Data Used to represent points, lines, and areas All are represented using coordinates One per point Areas as polygons Straight lines between points, connecting back to the start Point locations recorded as coordinates Lines as polylines Straight lines between points

Vector Data Point Number X,Y Coordinates 1 2,4 2 3,2 3 5,3 4 6,2 Points Point Number X,Y Coordinates 1 2,4 2 3,2 3 5,3 4 6,2 Line Number X,Y Coordinates 1 1,5 3,6 6,5 7,6 2 1,1 3,3 6,1 7,3 Polygon Number X,Y Coordinates 1 1,5 3,6 4,4 2,2 1,5 2 6,6 8,7 8,3 7,3 6,6 1 3 2 4 Lines 1 2 Polygons 2 1

An area (red line) and its approximation by a polygon (dashed blue line)

Raster vs. Vector Smallest addressable unit in Raster is a cell and in Vector is a point. Raster can represent both objects and fields, Vector has difficulty representing fields. Vector can represent the true dimensions of a object, Raster will always distort an object. Vector makes better graphics. Easier to write algorithms and integrate data in raster

Raster vs Vector Volume of data - raster becomes more voluminous as cell size decreases Vector handles tabular data more effeciently Source of data- remote sensing, elevation data come in raster form Vector used for problems where parcels and lines are issues (AM/FM, Property Mgt) Raster is better for fields and integrating data – used in modeling

Types of Attributes Level of Measurement Nominal – Name (land cover class) Ordinal – rank (a ranking of habitat) Interval – no true zero (Celsius temperature) Differences make sense Ratio – true zero (Kelvin temperature) Ratios make sense Cyclic – (wind direction)