2. Objectives
• Be able to describe what a GIS is and what it can do
• Be able to describe the 2 types of geographic data
• Understand the relationship between attributes and
geometry
• Know the different spatial data formats used in GIS
• Know where to get free base data and ancillary data
3. What is a Geographic Information System (GIS)?
A GIS is an integration of 5 basic components
5. Capturing Data
GIS
DATA
Paper Maps
-Drawn by hand
-Surveyor data
Satellite Images
-Google Earth
-Microsoft Virtual Earth
Global Positioning System (GPS)
-Latitude/longitude coordinates
-Routes/lines
6. Representing geographic features: two data models
There are two basic types of geographic data
Vector takes discrete
measurements along the
surface of a feature
Raster assigns square
cells to real-world entities
7. Geometry of vector data
• A point is represented as one coordinate pair
• A line is two or more points connected
• A polygon/area is at least three points that start and end at
the same place
6.8000 ° S, 39.2833°
1.2833° S, 36.8167° E
6.8000° S, 39.2833°E
Nairobi
Dar es Salaam
Dar es Salaam
0.157 ° N, 33.033°E
3.017° S, 33.033°E 0.452° S, 34.63°E
Lake Victoria
8. Components of vector data
• Attributes are text or numerical information that
describe the features
• Geometry are the shape of the features
10. Features
• Each record (row) in the attribute table is called a
feature
Selected feature (.DBF) Northeastern Region
Kenya Region Feature (.SHP)
11. Raster: Images and grids
• Rows and columns of equally-sized cell
– Each cell stores a value
– Detail depends on cell size
• Can be stored in various image file formats including:
JPG, IMG, TIF, GIF, BMP, etc
17. Ancillary Data – Background data from 3rd parties
• Humanitarian Response COD-FOD Registry
(http://www.humanitarianresponse.info/applications/data)
– Administrative Boundaries
– Populated Places
– Transportation Network
– Schools, wells, etc. (sector specific)
• GADM Database of Global Administrative Areas
(http://www.gadm.org/ )
– A spatial database of the location of the world's administrative areas
• DIVA-GIS
(http://www.diva-gis.org/Data)
– Download country level data for any country in the world: administrative
boundaries, roads, railroads, land cover, etc.
18. Review
• Two main types of geographic data:
- Raster
- Vector
• _________ are information that describe the features
while _________ are the shape of the features.
• Spatial formats include:
-Shapefile (includes tabular data)
-Raster
-KML
• Free ________ data and _________ data is available
on-line and may be added to your map directly from QGIS
Attributes
geometry
ancillarybasemap
Yesterday some of you asked where does the data come from? This session is designed to give you an introduction to GIS and GIS data sources.
Following the presentation, you will be working in your case study groups and begin working with QGIS software to add and explore data.
You can take notes in your participant guide on page 37 while I give the presentation to capture main ideas and to jot down any questions that you may have.
After objectives ask participants what GIS is to them before moving to next slide.
Ask participants what GIS is to them.
GIS stands for Geographic Information Systems. A GIS is a system of people, hardware, software, data, and procedures.
GIS databases are different from a typical database because geographic data correspond with places, not people or accounts.
In other words, all GIS data has a spatial or geographic component, and that’s what makes the technology so powerful.
This presentation will focus on the data component. Point on slide.
A GIS is more than just map making. A GIS allows us to capture, store, query, analyse, display, and finally output different kinds of data.
In this way, a GIS is comparable to a factory that transforms raw material into something more valuable.
GIS is a horizontal application, which means that it may be applied to many different types of fields, including military, road building, humanitarian aid, and even public health.
When working on a GIS project, one of the first decisions that is made is how the data will be captured and whether you are going to collect your own data, use existing data or a combination thereof.
GIS data comes from different sources, including data captured from GPS receivers, satellite images, or hand drawn paper maps., which then of course need to be converted to electronic format.
In GIS, there are two main ways to represent objects on the surface of the Earth: vector and raster.
A raster data set is made up of a grid of square cells or pixels where all the cells are the same size.
A vector data set is captured by taking separate point measurements along the surface of the feature.
I will talk in more detail about each type of data in the coming slides.
Let’s first talk about the vector data model.
The Vector Data is based on the assumption that the earth's surface is composed of discrete objects such as trees, rivers, and lakes.
As such the vector approach is well suited for mapping objects with well defined edges, such as highways, pipelines, or property parcels.
Vector data views features on the surface of the Earth as either a point, line, or polygon (area).
A point is one coordinate pair. Points can represent a town or a hospital, for example.
A line is two or more points. Lines can represent a road or a river, for example.
A polygon is 3 or more points that start and end at the same place. Polygons can represent a country or region, for example.
The most common file format for working with vector data is called a shapefile.
And we cannot combine different geometry into one shapefile.
Vector data consists of attributes, which describe the characteristics of the data and geometry, which describes the shape of the feature.
Now, when I say feature, I am talking about the object that we are mapping, whether it’s hospitals, roads, or land parcels.
So, imagine you are standing on the top of a hill. Looking down, what do you see? Houses, roads, trees, rivers.
Each one of these objects would be a feature when we represent them in a GIS Application.
And here we see lines, points, and polygons in the photo.
As I mentioned earlier, shapefiles are the most common format for exchanging GIS datasets.
Shapefiles store the dataset’s geometry, tabular data, as well as the projection.
So here’s how a shapefile looks in Windows Explorer.
Here we can see here that a single shapefile is actually composed of several computer files.
Depending on how the shapefile was created, there may be up to seven separate files that comprise the shapefile dataset.
How many does files make up this data set?
Explain that dbf hold tabular data while the shp hold the attribute data in a shapefile.
Just a few words of caution in the management of shapefiles.
1. If you want to share a shapefile dataset, all of the associated files must be transferred or the dataset will be unusable.
2. Caution should be used if you want to move shapefiles from their original location in the directory because most GIS software programs save only the file path to a shapefile, not the actual data itself.
Here we can see how each record or row in the attribute table shown here relates to the each geographic feature in the Kenyan region feature data set.
The Northeast Region feature of the Kenyan region polygon features is highlighted in yellow on the map and the related record is highlighted in blue in the attribute table.
The features are stored in a .shp format whereas the tabular data is stored in .dbf format.
So, can anyone tell me how many features the Kenyan region data set has? 8. Why?
Can anyone tell me how many records or rows are in the attribute table? 8. Why?
Now, let’s talk a little bit about the second GIS data type: raster data.
One important difference between raster and vector data is that if you zoom in too much on a raster image, it will start to appear ‘blocky.”
That’s because raster data set is made up of a grid of square cells where all the cells are the same size.
Each cell has a given value, depending on the type of dataset.
These cells are squares that are organized in rows and columns to form a rectangular or grid dataset.
The resolution of the image is determined by cell size.
Remember that vector data is most commonly stored in the shapefile format.
Raster data, on the other hand, may be stored in various formats, including jpg, img, tif, gif, and bmp.
So you may ask yourself, how can I know the best type of data format for my data? Well, the answer depends on what kind of data that you have.
For example, a raster strategy is a smart choice for representing data that lack clear-cut boundaries, like terrain elevation, vegetation, and precipitation, or temperature.
One thing to keep in mind is that raster data sets take up a lot more space on your hard drive than vector data and require more processing power than vector data.
This example from Nigeria displays the difference between satellite images of two different resolutions.
The higher the resolution, the smaller the cell size.
Here’s two examples of raster data sets: a digital elevation model that captures elevation values for the world as well as a map showing historic climate data around the world.
QGIS can handle data in multiple formats. And we talked already about shapefiles (or vector data), raster data, and tabular data.
But QGIS can also handle other formats, such as KML or keyhole mark up language, which is a kind of vector format that GoogleEarth uses to represent places on the surface of the Earth.
Like the shapefile format, each place always has at least one longitude and a latitude coordinate pair.
GiIS allows us to bring it all together by overlaying different kinds of data, a GIS allows us to interpret data and to understand relationships, patterns, and trends.
Over lay different kinds of layers one on top of another to better understand problems and issues
Turn data into knowledge.
In this way, a GIS can help us answer certain kinds of questions by comparing how layers relate to one another such as:
where is an entity located and what is its extent?
what are the characteristics of the entity located there?
when were the entity’s location, extent, or characteristics measured? And how has the entity changed over time?
However, it is important to keep in mind that decision support tools like GIS are no substitutes for human experience, insight, and judgment.
The base map is the bottom layer of information on your map that provides background reference information such as roads, landmarks, and political boundaries, onto which other thematic information is placed.
There is a wide variety of high-quality, freely available base data., such as world imagery, world street map, shaded relief, and topographic maps
Choose the basemap best suited to your map’s purpose and message.
These layers may be added to your map directly from QGIS. We will show you how to do this in a demonstration later.
A map depicting background reference information such as landforms, roads, landmarks, and political boundaries, onto which other thematic information is placed.
Ancillary data is reference data to support your map’s purpose.
For example, you may want to add major cities or rivers, roads to your map to add context and to help orient the map reader.
Ancillary data can also be used to assist in analysis and classification.
Humanitarian response COD-FOD registry is provided by UN OCHA to support humanitarian operations globally by offering sector specific data as well as more general data.
GADM is a global database of the world’s administrative areas in GIS format and contains an extensive set of attributes for each spatial feature:
DIVA-GIS is a free software package that was created primarily for analyzing and mapping biodiversity data, such as the distribution of species or other point-based patterns.
So just to review a bit:
What are the two main types of geographic data?
Vector data attributes are text or numeric information that describe the features whereas geometry is the shape of the features.
Spatial data formats include: shapefiles (which are vector data), raster (cell based data), table, kml (google earth format)
Look for free base map data from OpenStreet Map, Google, Bing and other sources online.
Free ancillary data is also available over the internet to provide context to your map.
