GIS Spatial Data Types

GIS Spatial Data Types

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In GIS, there are 2 basic spatial data types representing the real world:
Raster and Vector

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The Real World 

In this example, our landscape consists of

• rolling hills
• grassland
• lake
• rivers
• forest stands
• marsh

It can be represented as a series of raster layers and/or vector layers.

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Raster Data
In the raster data model, land cover is represented as:

• single square cells

Each cell will have a value corresponding to its land cover type.

Raster data are good at:

• representing continuous data (e.g., slope, elevation, chemical concentrations)
• representing multiple feature types (e.g., points, lines, and polygons) as single feature types (cells)
• rapid computations ("map algebra") in which raster layers are treated as elements in mathematical expressions
• analysis of multi-layer or multivariate data (e.g., satellite image processing and analysis)
• hogging disk space

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Vector Data
In the vector data model, features on the earth are represented as

• points
• lines / routes
• polygons / regions
• TINs (triangulated irregular networks)

Vector data are good at

• accurately representing true shape and size
• representing non-continuous data (e.g., rivers, political boundaries, road lines, mountain peaks)
• creating aesthetically pleasing maps
• conserving disk space

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Different types of data are
stored in different
file formats.

 

images © ESRI

Download free shapefile maps

Thousands of shapefile maps can be downloaded for free from the following websites, including country shapefiles, shapefiles at province or state level, and other administrative boundaries maps. The shapefile (or ESRI shapefile) format is a geospatial vector format and one of the most common (if not the most common) map format.

Shapefile maps can be loaded into StatPlanet Lite and StatPlanet Plus to create interactive shapefile maps (see also: setting up a shapefile map). Please note that you need to download the map files in the ".SHP" format to load them into StatPlanet.

• Natural Earth - world maps - Excellent world shapefile maps with national boundaries, first order admin (provinces, states, etc.), disputed areas, coastlines, populated areas and more at various scale levels.
• Map Library - Country shapefile maps in the public domain. The World Bank alternative below sometimes has better maps, but not for as many countries.
• United Nations - UN-SALB (Second Administrative Level Boundaries) data set project. These maps are free but copyright rests with the UN. The maps are validated by the National Mapping Agencies (NMA) of each UN Member State. This website requires registration to download the maps.
• Geocommons - A huge database of user submitted maps. Search for any kind of map, then downlooad the shapefile as follows: click on 'Details' (not 'View map'). Select a dataset listed below. To download the shapefiles, select the 'Shapefile' button shown to the right of the map.
• World Bank - Country shapefiles can be downloaded through the World Bank's Mapping platform as follows: Click on 'explore countries' to select a country, click on 'download' in the top-right corner, and select 'Indicators (shapefile)'. This option is not available for all countries.
• MapCruzin - A great source for all kinds of shapefiles.
• Open Street Map - A growing collection of shapefile map downloads by continent, region and country. These maps are community edited and are not always complete. Shapefiles by country are also available from VDS tech.
• US Census Bureau - USA County and District Cartographic Boundary Files.

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GIS Training with QGIS - Exercise 1: Vector layers and Symbology

Exercise 1: Vector layers and Symbology
 1. Install QGIS
 a) Windows users should download the OSGeo online installer (requires an Internet connection for the
installation) from  http://download.osgeo.org/osgeo4w/osgeo4w-setup.exe. Run the installer, and
choose “Advanced” setup. From the software list select to install  qgis-unstable. Verify that GRASS
is selected for installation, and choose all the gdal16 software libraries.
 b) Ubuntu Linux users can find ready-made packages from the UbuntuGIS software repository:
Details for adding this repo here: http://trac.osgeo.org/ubuntugis/wiki/UbuntuGISRepository
 c) MacOS users should install packages and required frameworks from:
http://www.kyngchaos.com/software/qgis
 d) Open a new folder Geodata and copy all the practice data layers from the CD into this folder.
 e) Make a subfolder “Exercises” under Geodata and save all work there.
 2. Start QGIS
 a) Re-arrange toolbars to find all buttons easily
 b) Disable all plugins except GPS, Delimited Text, GRASS, Quick Print.
 c) Under Settings->Options be sure that the CRS is set to “Prompt for CRS”.
 3. Load data
 a) Load the background tiff file HYP_HR_SR_W.tif from  the naturalearthdata.com directory
under Geodata\raster.
 b) Load all shapefiles from the naturalearthdata.com directory under Geodata\vector.
 c) In the Project Properties, set the projection to WGS84 (EPSG:4326).
 d) Choose “Unique Value” for the classification of the layer “states_provinces”. Use the ISO
column to give each country  a different outline color for its provinces. Set all the fill colors to
“None”.
 e) Color the rivers and lakes layers blue.
 f) Add labels to the populated_places layer. Use the Name_Town column as labels, and place the
labels below the points.  Use scale dependent rendering to label the layers only when zoomed in to a
scale of 1:3,000,000 or closer.
 g) Choose Graduated Symbol for the populated_places layer to display the towns with different
size symbols based on the POP_MAXI column. Enter a value of 4 in Number of classifications and
click Classify. Then change the symbol sizes such that larger populations are displayed by bigger
symbols.
 h) Display the countries layer with 70% transparency, setting a separate color for each country, and
thick dark outline.
 i) Zoom to Bosnia-Herzegovina and save a bookmark. Zoom to Serbia and save another bookmark.
 j) Save the QGIS project as Geodata\Exercises\Balkans.qgs
 4. Check the project
 a) Close QGIS and reopen the project (File->Open Recent Projects).
 b) Verify that all symbology has been saved and that the bookmarks work.