NRSC 2230 / GEOG 2750

Components of a GIS

Overview

This topic examines the general structure of an geographic information system and looks at the issues related to the characteristics of the data used in the system.

Learning Outcomes

Upon successful completion of this topic you will be able to:

  • Define a GIS
  • Describe the components of a GIS
  • Describe the components of spatial and attribute data
  • Describe the graphic entities (geo-entities / features) used to represent spatial data and the spatial attributes associated with each
  • Discuss the implications of feature resolutions and tolerances

What is GIS?

GIS is a computer based technology that incorporates geographical features with tabular data in order to map and analyze / assess real-world questions that relate to resource management.  A GIS is distinct from other information systems e.g. financial, management etc, in that much of the data used have a spatial component, in other words, data that is in some way referenced to locations on the earth. Coupled with this data is usually tabular data known as attribute data.  Attribute data are generally defined as additional information about each of the spatial features.

Activity 1 – Search for GIS Definition

Search the internet for a definition of GIS. You will find many of them. Post the definition that best describes GIS to you, on your personal Blog. Name this entry “My GIS Definition”.

Activity 2 – Comment on GIS Definition

Go to the blog of at least one other member of the class and post a comment on their preferred definition of GIS.  E.g. Do you fully agree or does it not take something important into consideration?

Spatial vs Non-Spatial data

Our definition of GIS refers to Spatial data.  Spatial data contain two broad groups of properties, geometric properties and topological properties.

Geometric properties relate to the measurement of points, lines, areas and surfaces that are used to represent natural features.  These geometric properties include position, length, direction, area, volume etc.

Topological properties relate to the relational characteristics that do not change even if the geometric properties change.  These topological properties include connectivity, inclusion and adjacency.

Non-Spatial data are said to have the properties of value and time.

Note:

All Mapping / GIS software store information regarding the geometric properties and the non-spatial properties of data. Only some software packages (e.g. ArcGIS workstation) store data regarding the topological properties of data.

 

Activity 3 – Explain geometry and topology

Use your favourite browser to research the terms Geometry and Topology in the context of Spatial Analysis.  Write a short post in your blog outlining how you would explain these terms (using examples) to a colleague who is not or has not taken a GIS course.

Activity 4 – Comment on the post in Activity 3

Go to the blog of at least one other member of the class and post a comment on their posting.

Components of a GIS

A GIS is much more than software. When reviewing a GIS one must look at the technology, the data used, the rules regarding the operation of the system, the personnel who operate the system.

Activity 5 – Search for GIS Components

Using your favourite internet search engine (Google etc.) find at least two descriptions of the components of a GIS.  In your blog write a brief comparison between the description provided in our discussions and the description found in one of these internet search results.  Are they saying different things?  Are they using different terms?  What are these terms? Name this entry “Components of a GIS”.

Activity 6 – Blog entry “Our Local GIS”

Write an entry in your blog that describes the GIS in your lab / office.  Please write about the technology ie. hardware and software (remember technology can be non-computerized), the spatial and attribute data you use, how these data are used and are there any data / technology specialists in the office.  Name this entry ” Our local GIS”.

Geo-Entities

Geo-entities are graphic shapes (points, lines, areas, surfaces) used to represent geographic features and they are generally described in terms of dimensionality.

Note:

The GIS software allows us to describe each geo-entity through an non-spatial attribute table.

The following paragraphs examine the characteristics of these entities and the types of spatial queries we can make using them.

Points are used to represent features with single x,y coordinates e.g hydrant, light pole etc. and are said to be zero-dimensional. The types of spatial questions we can ask of point feature are:

  • Does it exist?
  • What is its location?
  • Is it close to anything of interest?

Lines are used to represent linear features e.g. rivers, roads etc. and are said to be one-dimensional.  The type of spatial questions we can ask of a line feature are similar to those we can ask of a point and they also include:

  • What is its length?
  • What direction does it follow (orientation)?
  • Is it straight or does it contain many curves (sinuosity)?

Areas (Polygons) are used to represent area features e.g Lot boundaries, City boundaries etc. and are said to be two-dimensional. The type of spatial questions we can ask of an area feature are similar to those we can ask of a point they also include:

  • What is its area?
  • What is its perimeter?
  • Is it isolated or connected to another area of interest (e.g. are the cutblocks adjacent)?
  • Are there interior holes in the area ( e.g. are there lakes on an island)?
  • Is it overlapping another area of interest (e.g. does the moose habitat overlap the grizzly habitat)?
  • Is the area compact or is it a long thin sliver?

Surfaces are used to represent features in three dimensions e.g. terrain, sub-surface structures etc. and are said to be three-dimensional. The added dimension allows us to ask spatial questions such as:

  • What is the volume beneath the surface (e.g. earth removal from cuts in road construction)?
  • What is the slope between specific points along the surface (e.g. slope variation along a hillside)?
  • What is the aspect of a specific part of the surface ( e.g. direction to which the surface faces)?
  • What is the profile between specific points on the surface (e.g. the general shape of the landscape between the two points)?
  • How much of the surface can we see between two specific points (sightlines / visibility)?

You may see the terms multi-point, multi-line and multi-polygon in your readings.  These terms refer to geographic features that are represented by more than one basic geo-entity.  For example, a single branch of a river may be represented by a multi-line or an area with many lakes may be represented by a multi-polygon.  In each case the multi feature is linked to one record in the related data table.

Activity 7 – Researching Feature Class Basics

Use your favourite browser to search the ArcGIS Resource Center Desktop 10 Help on the topic “Feature Class Basics”.  Provide a post on the following topics, in your blog:

The system uses X,Y resolution and X,Y tolerance values to provide adequate data accuracy and precision. Which of these do you think is more important and why.  Title this post Accuracy and Precision.

Explain how M values would be of importance in analysis in your field of enquiry .  Title this post M values.

Activity 8 – Comments on “Accuracy and Precision” post

Go to the blog of at least one other member of the class and post a comment on their contribution.  E.g. Do you fully agree or does it not take something important into consideration?