Modelling the world in G.I.S. Flashcards
How can we model reality?
We can use vector geometry.
What is a point?
The simplest form of vector geometry which has an x and y coordinate to say where abouts it is in the world.
A point can also have a third piece of information when looking at coordinates, what is this and what does it mean?
The z value and that is the height above sea level.
What can we use context for?
To give spatial information more meaning for example by adding a background map.
What could a point represent?
A house and we would denote a house symbol etc.
What do we use to give meaning to coordinates for interpretation purposes so people understand what information they contain?
Symbolisation
What could you model Newcastle as?
A point or as an area.
What is vector data modelling associated with?
Location where we encode more information, for example we could add the address of a house or postcode,
We can start associating this information for each house what do we call this?
Attribute information and you store it in an attribute table.
What is the attribute information for a house?
1) House number
2) Post code
3) House price
4) Council tax band
When could attribute data have a negative impact?
When information is shared about what time of the day you leave the house and when you come back, impacting privacy.
When do you get a spatial data layer?
When you start creating collections of features with a set of shared attribute types.
What do most coordinate systems consider when calculating spatial relationships?
They consider the world to be a two-dimensional grid, just like a piece of graph paper allowing us to measure between two points and use Pythagoras.
What happens due to the earth being globular?
When measuring larger distances you get problems because of the curvature of the earth.
True or false, “you can use all coordinate systems for large areas”
False, you can only use certain coordinate systems for small areas where we can consider the world to be flat.
What are latitude and longitude?
They are the measurement of an angle from the centre of the Earth, it makes distance calculation very complex and less accurate.
What is it called when point and attributes join together in sets?
A layer or a feature class.
What is another way we can model our data?
Is through thinking about it as a line.
What do we model a line as?
As a set of coordinates, but we don’t model the coordinates all the way along it we only store information whenever there is a change in direction.
Do the coordinates along a line have an order to them?
Yes, a start and an end and that is how we model any of those features such as roads and railways.
When we have a number of lines what do we have?
A feature class.
What does a feature class share?
Shares a geometry type and a set of attributes with everything else in that feature class.
What are maps for navigation all about?
Linear features
What can you do thematically to display a road?
Change the colour of it and change its widths depending on it.
On an ordinance survey map what is a minor country and what is a really small road denoted as?
Minor country road- yellow line
Really small road- grey line
What is a polygon?
It is an enclosed shape, an irregular shape and this is the other way in which we model any other geometry from using vector modelling.
What are the 3 ways we model geometry using vector modelling?
1) Points
2) Lines
3) Polygons
What could a polygon represent?
A building or agricultural fields or countries.
What must a map contain?
Must contain meta-data or relevant explanation.
How do we define a polygon?
By its boundary and we model it in the same way we model a line. For example start at one of the corner and every time we change direction we store a coordinate and we finish by storing another coordinate where we started.
What do we use to represent a quantity associated with an area or polygon?
Pie charts
How can we model the weather?
Start off with a square called a cell or a grid cell and in that cell we add one piece of information which is always a number. This square also represents a physical amount of area which is called the spatial resolution.
What happens as you increase or decrease your resolution?
You exponentially increase your storage requirements.
What is it called when we combine cells together?
Grid pattern
What is land use map a good example of?
Continuous information in the form of a category not a number.
What does a land use map say?
It say what the land is being used for e.g. agriculture, building or roads. therefore the number inside of the cell represents a category instead of a measurement
What is a digital elevation model?
Where the number represents the height of that grid cell above sea level.
How can we model the world?
Through discrete features; points, lines and polygons that we attach attributes to. And modelling continuous surfaces using grid cells. Where a cell either represents a quantity or a category.
Know what vector data and raster data looks like.
Look on lecture 3.
Define spatial resolution.
The length of one cell.
What does it mean if you say “a cell has a spatial resolution of 5m”?
One cell represents 5m in the real world.
Typically for Arial photography what is the spatial resolution?
Roughly 2cm.
Typically for satellite data what is the spatial resolution?
1m up to 50-100m.
What is raster data stored the same as?
Image data
What is a picture?
A set of numbers, where the numbers refer to a grey scale.
What do we use for coloured images?
The RGB index (Red, Green, Blue).
What are all images made up of?
3 layers; a red layer green layer and a blue layer and there is and a value for each layer. The values go between 0 and 255 for each layer.
Why do we sometimes what a single intensity layer?
Because vegetation shows up better under certain colours.
