GIS Basics - General Introduction of Geographic Information System
GIS Basics

1. Themes or Layers

Maps produced traditionally or by automation involved the separation of data into layers: each contained different types of features: e.g. rivers, roads, etc. These are then combined to form a map where layers are printed in different colours. To present all the data for an area might require the production of several maps, as they could not all be printed together.
In GIS, data are divided into layers or themes, divided by type but for the dual purposes of display and/or analysis.

The database can be divided into as many layers as is necessary, where each layer contains one characteristic such as soils, land use, drainage, etc. The layers should 'overlay' each other perfectly (as a result of georeferencing) and enable analysis between layers.

Figure 2-1 : Data layers
GIS tutorial dan Remote Sensing of facegis.com


2. GIS feature class ('geometry') types

There are three main types of 'map' or vector' data:

a. Points:

  • have no length or area at the given scale
  • have a single X, Y coordinate
  • represent a feature that is too small to be displayed as a line or area

b. Lines:

  • have length but too narrow for width to be shown, e.g. a creek, small road
  • a set of linked coordinates as nodes and vertices

c. Areas (polygons):

  • have an area that is given by the lines (arcs) that make the boundary.
  • are used to represent features that have area (e.g. lakes, large cities, islands)

There are other (more complex) types, especially annotation (text)


A GIS layer does NOT combine points, lines and polygons: each type requires a separate layer

These data are also on separate layers in traditional map printing topomap example


3. GRIDS (Raster data)

Vector data consist of points, lines and polygons formed from lines joining x and y coordinates.

Most map data are vector and stored more efficiently as vectors.

Raster data are organised in grids, one value per grid square. All photographs and images are raster. Elevation data are usually raster, other layers can be raster but are more often vector.
Any layers can be made into raster format for raster analysis (see labs 15-16). Most GIS software enables the use of both data types (models). example

Figure 2-2: A simplified example of the difference between raster data and vector data

GIS tutorial dan Remote Sensing of facegis.com

GIS tutorial dan Remote Sensing of facegis.com
In a more complex example, this image shows both raster data (satellite image of UNBC and area) and vector data (Cranbrook Hill Greenway).† This image is at normal size (no zoom).
GIS tutorial dan Remote Sensing of facegis.com
When we zoom in 16X from the previous image, the difference between raster and vector data becomes more apparent. The raster data is defined by an individual colour (representing a data value) for each grid cell. The vector data remains a solid red line with the same width.


4. GIS spatial features and attributes

GIS data contain these two components:

    Spatial data: allows us to answer the question "where is it?"
    • Stored as x and y coordinates ( = 2 columns of data)
    • Gives information on the location and shape of features
    • Gives information between geographical features e.g. proximity

    Attribute data: allows us to ask the question "what is it ?"

    • stored in a data table, rows (records) and columns (items)
    • entries may be text or numeric e.g. marsh, high / low, 2,3
    • Give information on the nature and qualities of features

A few more details on attribute tables:

    • Every vector layer MUST have an associated table
    • These are linked to spatial data by a feature code number ('id').
    • Attributes are stored in columns as 'items'
    • Rows display the attributes for each feature and are known as 'records'

    Record ITEM 1
    (e.g. name)†
    ITEM 2
    (e.g. age)†
    ITEM 3
    (e.g. height)†
    I.D. #†
    †1 Spruce 50 30.5† †1†
    †2 Pine† 10 5.5† †2
    †3 Aspen† 20† 10.0† †3

These can be stored in an associated GIS database (within the software), an external database (e.g. Oracle), a standard database or spreadsheet (Access, Excel), or a text file (e.g. .csv:† comma separated values)

Perhaps the prime reason GIS has become dominated by vector data is the need to manage attributes.

This shows an example from forestry:†† table†† ††† map legend


5. Scale

It is undesirable to represent or 'model' the Earth's surface at its full size.

GIS data and maps show a scaled portion of the Earth's surface.

    Scale = the amount of reduction (expressed as a ratio)
    e.g. 1:10,000 => a reduction in size / detail by 10,000 times
    conversion to scale statement †=> 1cm = 10,000cm (or 1cm = 100m)
A larger scale is thus reduced by a lesser amount. Hence 1:50,000 is a larger scale than 1:250,000.

Fig 2-3 : Common Map & Data Scales

AGENCY†
SCALE
DESCRIPTION
ANALYSIS
International
1:10million†
Very small†
Global
Federal /Int.
1:1,000,000†
Small
National
Federal†
1:250,000
Medium†
Regional
Federal†
1:50,000
Large
Local
Provincial†
1:20,000
Large†
Local†
Municipal
1:2,500
Very large
Cadastral

Data levels 1-5 (very small to large scale):† www.mapplace.ca or www.lrdw.ca

Data level 6 (municipal data):† pgmap.princegeorge.ca

Data collected at a specific scale are suitable for mapping and analysis only at similar scales

  • At smaller scales, large scale data† are too complex (but could be generalised)
  • At larger scales, small scale data are too generalized (detail cannot be 'added')
As scale is reduced ->† fewer elements, fewer details; more areas become points and lines


6. Accuracy & Precision

These two words are often incorrectly used interchangeably.
    Precision: How exactly a location is specified (relates to the exactness of the method used).

    Accuracy: How close recorded location is to true value (relates to the exactness of the result) .

Traditionally, suitable precision was approximately '0.5mm' on the map† (the smallest distance that can be measured, or two features to be seen as separate).

Fig 2-4

SCALE
PRECISION (RESOLUTION)
ACCURACY (DETECTION)
1:1,000,000
500 metres
1000 metres
1:250,000
125 m†
250 m
1:100,000
50 m
100 m
1:50,000
25 m
50 m†
1:20,000
10 m
20 m†
1:5,000
2.5 m†
5 m†
1:1,000
0.5 m
1 m†

* As you can see from this table,† precision is usually finer than accuracy (i.e. it's easier to be precise than accurate).†† Technically in GIS†databases and mapping, it is possible to be more precise and accurate than on hardcopy maps, but this depends on the quality of the data.

Some GIS use 'double precision' enabling up to 6 decimal places (of a metre) e.g.1234567.123456 (metres or sq.m)

Do NOT repeat decimal places given in the GIS unless the data accuracy warrants it!!!

Still confused? Get more information at this†GIS tutorial dan Remote Sensing of facegis.com


7. Spatial coordinates

Georeferencing or the system for assigning coordinates is most simply done globally using the graticule = latitude and longitude. Handy for data storage, but not for display as it is not rectangular.

More common is the Universal Transverse Mercator (UTM) system

  • Coordinates are given by 'eastings' and 'northings'
  • Easting (6 digits): relative to CM = 500,000 (metres)
  • Northing (7 digits): relative to Equator = 0 (metres)
  • Equator to Pole = 10,000,000 metres††

UTM works well for local areas e.g. City of Prince George

Source: http://www.gis.unbc.ca/courses/geog300/lectures/lect2/index.php