PL254 INTRO. TO GIS & MODELS IN PLANNING

1. PL254 INTRO. TO GIS & MODELS IN PLANNING UNIT 2:  LAYERS & FEATURES IN DIGITAL MAPPING

2. Meaning of GIS What is GIS? • GIS = Geographic Information System(s) • It is defined as: “a computer-based system to aid in the collection, maintenance, storage, analysis, output and distribution of spatial data and information” (Paul Bolstad, 2005)

7. Layers, Features & Tables of Content What is a “Feature”? • Any geographic (spatial) object that can be displayed on a map • Give examples

8. Layers, Features & Tables of Content What is a “Layer”? • A collection of Features or “Surfaces” that are alike • For example we can have: • A layer of settlements in Upper East Region • A layer of rivers in Ghana • A layer of government lands in Accra • A layer of oceans in the form of a “surface”

9. Layers, Features & Tables of Content What is a “Table of Content”? • It is a digital list of layers displayed on a map

10. Shape, Location and Attributes of a Feature Features have Shapes • “Vector Features” can take various shapes. These shapes can be classified according to three main geometric forms: • Points • Lines & • Polygons

11. Shape, Location and Attributes of a Feature Polygon: • Used to show geographic objects (features) that have boundaries. A polygon has a location, a diameter and an area • Examples: districts, lakes, parcels of land, built-up area of a township, etc

12. Shape, Location and Attributes of a Feature Line: • Used to represent elongated features that are too narrow to be polygons. A line has a location and a length but has no width or area • Examples: roads, streams, etc

13. Shape, Location and Attributes of a Feature Point: • Used to show features that are too small to be polygons or lines. A point has a location but has no dimensions—no length, width or area • Examples: settlements, standpipes, schools, health facilities, etc

14. Shape, Location and Attributes of a Feature What do you understand by “Location of a Feature”? • The geographical position of a Feature • Usually determined by a Coordinate System • E.g., a coordinate may be measured as: • Horizontal (X-Axis) distance and Vertical (Y-Axis) distance from an origin (0,0) in an X-Y plane OR • Latitudinal and Longitudinal (angular) values

15. Shape, Location and Attributes of a Feature • The location of a point is represented by a pair of X-Y coordinates • A straight line has two pairs of X-Y coordinates • What about a non-straight (winding) line? • The location of a polygon is represented by the coordinates of its vertices

16. Shape, Location and Attributes of a Feature What do you understand by the “Attributes of a Feature”? • The characteristics of a feature • E.g., attributes of a settlement: • Pop. size, pop. growth rate, status as district capital, a market town, etc • GIS can display virtually any attribute but it is up to the GIS user to decide which attributes are needful

17. Shape, Location and Attributes of a Feature Attribute Tables • A spreadsheet-like table attached to a layer showing the attributes of the features displayed by the layer • It has rows (Records) and columns (Fields) • The Records show Features in the layer while the Fields show the Attributes of each Feature • The Attribute Tables are not seen on the map but are linked to most of the information displayed on the map

18. Shape, Location and Attributes of a Feature Surfaces • Special kinds of features used to depict things such as topography (slopes, elevations, etc), temperature and rainfall patterns, etc • Often represented as raster features—consisting of cells with varying values