Spatial analysis: Vecotr analysis Flashcards
lecture 16
define spatial analysis
the process of applying analytical techniques
to geo-referenced datasets to extract or generate new
geographical information.
what is spatial analysis
- A method of analysis is spatial if the results depend on the
locations of the objects being analysed
▪ Move the objects and the results change
▪ Results are not invariant under relocation - Spatial analysis requires both attributes and locations of objects
▪ Remember, a GIS has been designed to store both
Prior to performing spatial anlysis what needs to be done?
NB NB NB NB
- All extensions are checked especially the “spatial analyst” extension
- All the data layers MUST be in the same coordinate system
- Data MUST be in a Projected Coordinate system when using the
Proximity tools. Why? Because you work with DISTANCE
same geographic co-ordinate system and projected system.
What is vector analysis?
- Spatial Analysis investigates the contextual relationships in
mapped data. - The spatial analysis of vector data is often referred to as “vector
analysis” - Vector analysis refers to common spatial analysis operations that
can be performed on vector data.
Examples of vector analysis
- Buffering
- Overlay
- Distance
- Pattern Analysis
- Feature Manipulation
What are buffering?
Proximity tools.
Buffers are used to create boundaries at an equal distance in all directions from a feature
* Buffers are typically created as polygons because they
represent an area around a feature
* can be point, line or polygons
– e.g., to determine areas impacted by a proposed highway
– e.g., to determine the service area of a proposed hospital
– e.g., to determine zones of different noise intensity around an airport
There are different variations of buffers:
1. Buffer distance
▪ Vary according to the values of a given field
▪ Variance can also be caused by adjacent land-use (topology)
2. Multiple ring buffer
▪ Distance from the centre points increases, thus forming multiple rings
increasing in size outward
▪ Intervals of each ring is the same, But rings are not equal in size.
Boundaries of buffer zones may
remain intact so that each buffer
zone is a separate polygon, or
boundaries may be dissolved.
What is an overlay?
Combines the geometries and attributes of two feature layers to create the output.
* Geometry of output represents the geometric intersection/connection of features from the input layers.
* Each feature on the output contains a combination of attributes from the input layers, and this combination differs from its neighbours.
What are the overlay types?
POINT-IN-POLYGON
➢ Input: point layer
➢ Output: point layer with attribute data from polygon
➢ Example: Wildlife locations in veg zones
* LINE-IN-POLYGON
➢ Input: line layer
➢ Output: line layer; broken into two segments, each segment has attribute data from polygon
➢ Example: Soil underlying a proposed road
* POLYGON-ON-POLYGON
➢ Output combines geometry and attribute data from the two layers into a single polygon layer.
➢ Example: Association between veg and elevation.
What are overlay tools?
Union:
* Applies ONLY to polygon data
* Preserves ALL features from the inputs
* Area extent of output = input1 + input2
* Combines attributes of all inputs in the output
Intersect:
* Can be applied to points, lines and/or polygons
* Only features or portions of features which overlap in ALL layers written to Output
Symmetrical difference:
Applies ONLY to polygon data
* Features or portions of features in input and update features which do not overlap will be written to the Output Feature Class (opposite to INTERSECT)
Identity:
Output has the same extent as input layer
* Output includes the geometry and attribute data from the identity layer
* Input that overlaps Identity Features will get the attributes of those Identity Features.
* Identity or Minus uses [(input layer) AND (identity layer)] OR (input layer).
What are overlay errors?
In any two layers there will almost certainly be duplicate lines.
* Slivers are gaps between polygons.
* As a result large numbers of small sliver polygons will be created.
– these must somehow be removed.
– this is normally done using a user-defined tolerance.
* To remove slivers, ArcGIS uses the cluster tolerance, which forces points and lines to be snapped together if they fall within the specified distance.
What is distance/proxomity?
- Distance or proximity analysis measures straight line (Euclidean)
distances between features - Can be used directly for data analysis
- Can also be used as inputs to data analysis: gravity model,
pattern analysis - Distance or Proximity tools include:
1. Near
2. Point distance
Distance and proximity tools?
- The Near tool calculates the distance from each feature in the input features to the nearest feature in the another layer, within the search radius.
You might use Near to find the closest stream for a set of wildlife observations, or the closest bus stops to a set of tourist destinations. - The Near tool can also add attributes:
- Coordinates of and
- Angle toward the nearest feature.
- Point Distance determines the distances from input point features to all points in the near features within a specified search radius.
- This table can be used for statistical analyses, or it can be joined to one of the feature classes to show the distance to points in the other feature class.
- Example: Distance from crime scene to police stations.
What is pattern analysis?
Refers to the use of quantitative methods for describing and analysing the distribution pattern of spatial features.
* Can be Random, dispersed or clustered patterns.
* Because pattern analysis could be a precursor to more formal and structured analysis, some researchers see it as part of data exploration.
* Examples:
* Spatial distribution of plant species OR Hot-spot analysis for mapping crime locations.
Can be Random, dispersed or
clustered patterns.
Modelling is the most advanced form of spatial analysis
ArcGIS anaylsis tools?
Spatial statistics examine the distribution of values associated with geographic features and make comparisons to a hypothetical random spatial distribution in order to answer location based questions in a
statistically sound way.
What is feature manipulation?
Tools are available in ArcGIS for manipulating and managing
maps and features in a database.
* There are numerous feature manipulation tools
* These Include:
1. Clip
2. Select
3. Split
4. Update
5. Erase
6. Append
7. Dissolve
8. Eliminate
