Fields Flashcards
What questions does field information answer?
Field information answers questions about the value of an attribute at any location and time of interest.
Field information answers questions on “what is here?” (E.g. What temperature is it in Santa Barbara today?”)
Where do attribute values come from?
Attribute values result from measuring or calculating a thematic property, such as temperature.
What are fields?
Fields capture a theme through an attribute taking a value at any location and time of interest.
E.g elevation as a field, and produces a simple model of terrain as a surface, though it cannot account for overhanging cliffs and natural bridges.
What shape are cells in field data?
Fields can be represented as regularly shaped cells in a coordinate system, and for 2D can be square, or hexagonal, or in 3D can be cubes.
Can be irregularly shaped cells, such as graticule cells which are delimited by parallels and meridians and can be irregular (e.g the Theissen polygons of each access point) this makes them unsuitable for global fields, which are better served by geodesic or hierarchical grids.
What characterises field attribute values?
Field attribute values are normally scalar, or vectors if thematically related values.
How do you gather attribute values?
They may be:
- Sampling the attribute at or within a location (elevation)
- Assigning a nominal value to the location (soil type)
- Counting samples or calculating a statistic (a count of insects or an average elevation
- Normalising a value by area and the location and /or the duration of time (population density or amount of rainfall in an hour)
What in general characterises all fields?
Every field is characterised by a field function, that maps locations and times to attribute values
Attribute value = f(location, time)
What does the field function map?
the region and time the attribute values take forms the domain of the field function.
What is the field domain?
The domain contains all locations and times for which the field function provides attribute values.
For example, a temperature field may be of interest for a particular country and day, leading to a domain of ‘USA yesterday’.
Attribute values for all locations at a given time are considered fields, but attribute values for only one location over time (time series) is not thought of as a field.
What are continuous fields?
Continuous fields have continuous field functions, mapping small changes in location or time in attribute values. This characteristic of many geographical phenomena expressed through tobler’s first law of geography.
E.g. temp near the weather station is similar to that at the weather station and similar to temp 10 mins before.
Land cover - non continuous as values abruptly change (land to water)
How are fields represented?
Fields are represented by representing their field functions. Field representations can be distinguished by how they capture the functional relationships between location, time and attribute values. In practice these relationships are represented separately.
How is the relationship between time and an attribute value represented?
The relationship between time and attribute can be captured through temporal screenshots of the whole field or through time values at al locations. The first is a snapshot, the second provides flexibility to record attribute values between different locations and different times.
What is the relationship between location and attribute values?
The relationship between location and attribute values can be captured in three ways:
- Through the complete set of location-value pairs
- Through a sample of location-value pairs together with an interpolation procedure for values at all other locations (and maybe times)
- Through one or more equations to calculate attribute values from locations and possible times.
What is the most common way of representing fields?
The most common representation of fields stores location-value pairs for all locations in the domain.
E.g satellite images represent fields through raster data, where the picture elements (pixels) are squares in an image coordinate system. After georeferencing images represent remote measurements of a thematic attribute such as infrared radiation or surface temperature, in a regular taster grid.
How are fields represented as samples with an interpolation procedure?
Interpolations are performed when a phenomena is observed at selected locations only.
Triangular irregular networks are formed from triangulation between sampled points which produces a triangular irregular network, with linearly varying elevation values within each triangle.
