Spatial analysis: Raster features Flashcards
Lecure 17
what is raster analysis?
spatial analysis of raster data
Use integer beased grids to represent discrete data and f,oaring-points to represent continuous data.
Integer rasters?
VAT tables (value attribute table)
Record stores unique values and the number of cells in the grid represented by that value.
Can have cells with no data = not 0!!
Floating-point grids?
No VAT
cells in grid can assume any value within a given range of categories.
No discrete categories.
Elevation data.
Raster analysis in ArcGIS
- Spatial analyst is activated
- Specifed raster environemnet set
Area can correspond to specific raster, area defined by min and max co-ordinates, combination of rasters.
- Analyses are performed using GUI tools
- Spatial Analyst Tools < Map Algebra
<Raster Calculator
What are raster operations?
Various spatial analyses can be perfomred on raster data:
1. Local
2. Neighborhood / Focal
3. Zonal
4. Global
What are local raster operations?
cell-by-cell operations
New raster from a single or multiple raster inputs
new cell values are computed by relating inout to output raster.
Local functions apply an indepndant calculation to each individual input raster cell - use Map Algebra.
Types of local raster operation
Reclassification
Multiple raster layers
What is reclassification?
- Creating new rasters through classification
- Recoding / transforming
- 2 ways:
- one-to-one
*range-to-one
purpose?:
Simplifying raster
Creating new rasters
Shows ranking
What are multiple rasters?
- Composting, overlaying, superimposing.
- Numeric data to calc output values
- No-data in input = no data in output
- Summary stats = max,min,ave, median,sum, SD
- use numeric or categorical data to calc output
- combine assigns a value output to each unique combination of inpout vals
What is focal raster operations?
Focal cell and set of its surrounding cells
Surrounding cells are sued for distance and directional relationship of focal cell.
common neighbourhood shapes = rectangle, circle, annulus, wedge.
Some cells may only be partially covered by defined neighbourhood.
General Rule include cell if its centre falls within defined area
focal cell marked with x
works on single rasters
function applys one calculation to all input raster cells within a neighbourhood.
Examples of focal operation?
simplification and generalizaion = reduces the level of cell value fluctuation.
smoothing = calculate the majority statistic using 3x3 neighbourhood assigning most frequent value.
Feature manipulation = uses range measure the different between min and max, high values indicates an edge.
Variety = tabultes the number of idfferent cell values in the neighbourhood and assigns a new value to the focal cell.
site selection = Ability to summarize stats in a
defined area
terrian analysis = Slope, aspect and surface
curvature measurements are all derived from
neighborhood operations using elevation data
What are raster zonal operations?
work with groups of cells of the same value or like
features
may be contiguous or non-contiguous.
are defined as a group of cells having the same value
Zonal functions apply one calculation to all input raster cells within each zone.
Summary stats and measures include: area, min, max, range, sum, mean,
standard deviation and median, majority, minority, variety [if the input raster = Integer]
zonal functions?
Useful for studies of landscape ecology
What are raster global operations?
Extended neighborhood operations
Global functions apply a calculation considering all cell values in the
raster
What are distance measure operations?
- Physical distance can also be called neighborhood or global operations
- Physical distance measure straight-line (Euclidean) distances away from cells (Via Pythagoras).
- Cost distance - considers time, cost, speed, etc
- “Buffers” source cells with wavelike continuous distances over the entire raster
- Reclassify can convert a continuous raster to one with discrete distance zones
What is the definition of distance?
The physical distance measures in cell units from each cell to the closest source cell.
Calculating distance surfaces and buffers?
allocation - closest source cell
direction - to the closest source cell
Other raster operation examples?
- Raster Data Management
– Mask: Is kind of like clipping
– Mosaic: Combining multiple rasters into a single raster - Data extraction (like CLIP in Vector data analysis)
- Data generalization
What are raster generalisations?
- Aggregate/ Aggregation
- Resampling technique that lowers resolution of raster.
- Calculates output cell value of all input cells that fall within the new (larger) output cell.
- Summary stats like min, max, range, sum, mean, median and standard deviation apply.
- RegionGroup -> aggregate per zone.
Map algebra:
Map algebra is calculated with the Raster Calculator Tool
output raster is always binary.
Calculations accross multiple rasters?
Multi-raster analyses is possible because of spatial registration multiple raster layers share the same X, Y coordinate space cell values are calculated across multiple raster layers to create a single output raster layer.
Map querying across various raster layers?
- Raster Calculator is easy to use and gives rapid results
- Results of overlay depend on cell size & relative precision
- Multiple rasters can be simultaneously queried (whereas only 2 vector layers can be compared in vector overlay)
- Output represents cells that meet and do not meet query criteria.
in Arc GIS
Map Query - Find cells where:
1.distance to streams < 300 ft and
2.elevation > 1500 ft and
3.timber volume > 60 mbf/ac
Result is a new temporary raster
Cells that meet all three criteria identified (value = 1)
Vector vs raster analysis overlay comparasin.
Vector:
* Intersection must be computed
* Vector overlay can only combine attributes from input layers.
Computation must follow overlay
* All attributes from input layers can be combined, then queried and analysed
* Efficient if large number of attributes share same geometry
Raster:
* Same cell size and area extent
* Many mathematical functions available through Raster Calculator
* Each input raster is associated with a single spatial variable, i.e. one raster for each attribute
* Computationally more efficient
Vector vs raster buffer analysis comparasin.
vector:
* Uses x-, y- coordinates in measuring distances
* More accurate buffer zones
* More flexible buffer options
– Multiple ring buffers
– Separate / dissolved buffers
raster:
* Uses cells in measuring distances
* Continuous distance measures
– requires additional data processing (reclassify/slice) to define buffers
– Difficult to create and manipulate separate distance measures
