Map Scale, Resolution and Data Models

Components of a GIS Map Maps can be displayed at various scales –Scale - the relationship between the size of features on a map and the size of the corresponding places in the real world Scale is most commonly expressed as a ratio –1:100,000 or 1/100,000 Verbal –One inch represents 2,000 feet (1:24,000) –One cm represents 20 km (1:2,000,000)

Copyright © 2006 by Maribeth H. Price Map scale Ratio of distance on the map to distance on the ground Dimensionless: cm or inches or mm… 1 cm on map = 100,000 cm on ground

Copyright © 2006 by Maribeth H. Price Talking about map scale A large denominator gives a small fraction  a small scale map. It shows a large area. A small denominator gives a larger fraction  a large scale map. It shows a small area ,000, , ,000

Large scale vs. Small scale Large scale maps represent smaller areas and thereby show greater detail –Typically large scale maps > 1:24,000 Small scale maps represent larger areas with less spatial resolution –Typically small scale maps < 1:500,000 Typical scales –Metric 1:10,000 or 1:25,000 1:50,000 1:100,000 –English System: 1:9,600 (one inch represents 800 feet) 1:24,000 (one inch represents 2000 feet) 1:62,500 (one inch represents (slightly less than one mile)

Map Scale

Copyright © 2006 by Maribeth H. Price Digital vs original scale Once in GIS data may be displayed at any scale, BUT Original scale of the map does impact the precision and accuracy of the data. Original scale 1:10 million Original scale 1:100,000 You should not display or analyze data at scales very different from the original source data.

Copyright © 2006 by Maribeth H. Price Units The cursor position on the screen may be read in several different types of units. Map units are determined by the x-y coordinates stored in the data file. –Usually they are in degrees, feet, or meters Display units can be set by the user, so that the coordinates may be viewed in any desired unit, such as miles. Page units show the location on the map page layout, usually in inches or cm.

Copyright © 2006 by Maribeth H. Price 2-9 Scale bar shows current scale as you zoom in/out Set map units and display units in layer properties Position bar shows current location of cursor Display units Page units

Copyright © 2006 by Maribeth H. Price 2-10 Display scale Visible range set less than or greater than a specific scale. Helps avoid clutter. 1:260,425 1:325,582

Copyright © 2006 by Maribeth H. Price Reference scale Reference scale sets the size of features relative to a specific scale. When set, symbols change size after zoom Reference scale not set Reference scale set Reference scale

Copyright © 2006 by Maribeth H. Price Don’t confuse these scale terms The map scale is the ratio of the map units to the ground units, e.g. 1:24,000. It changes when you zoom in or out. The display scale range controls whether a layer is visible only at certain scales. The reference scale determines whether symbols/text also change size when the map scale changes

GIS represents the real world Data Model – the objects in a spatial database plus the relationships among them (Bolstad 2002) Each spatial feature in our data model is represented by two distinct types of data: –Coordinate or Geometric –Attribute data

REPRESENTATION AND DATA STRUCTURES Coordinates and Attributes Bolstad 2002

Data Models Vector Data Model –Conceptualizes the real world using three different types of elements: Points Lines Polygons Bolstad 2002

Polygons Bolstad 2002

Data Models Raster Data Model –A conceptualization that uses a grid of cells to represent an area of interest. –Usually used to represent variables that change continually over an area i.e.: elevation, slope, depth, mean temperature, rainfall, etc. –Raster contains 1 value indicating a single attribute— i.e.: road type Only numeric attributes may be stored

Raster Data Model Bolstad 2002

Raster Data Model Bolstad 2002

Copyright © 2006 by Maribeth H. Price Continuous data Raster is the best way to store continuously changing values such as elevation Analysis faster and more flexible than vectors for many applications Some analysis only possible using rasters

Copyright © 2006 by Maribeth H. Price Impact of resolution Storage space increases by the square of the resolution Portraying large areas at high precision is problematic 90m resolution 10m resolution

Vector vs. Raster Bolstad 2002

Which model do I use? Raster Model Advantages –Represent frequent changes in space well –Other raster layers overlay easily due to grid nature –Raster data is “simpler” for a computer to handle –Best method for digital images

Which model do I use? Vector Model Advantages –Less memory intensive –Excellent for representing linear features –Best map representation for discrete objects –Allows for more accurate analysis on adjacency and connectivity