Intro. To GISLecture 6

Spatial Analysis

April 8th, 2013

Reminders

• Please submit your homework

• Project?

Spatial Features

Types of features in GIS

1) Point Nest Site; gas stations

2) Line Movement Path; River;

3) Polygon Home Range; Nest Plot

Note: When we talk about features we mean vector datasets

Spatial Features cont.

• Feature Class– Group of features of the same geometry– Can be filed in different formats

Shapefile

Spatial Analysis

• The application of operations to coordinate and related attribute data

• Maps are great, but this is the real power of GIS

• Spatial analysis is used to explore or solve a problem using a variety of geoprocessing tools

BASIC SPATIAL ANALYSIS TOOLS IN A GIS

database queries Selection/selection by location

Spatial joins basic statistics Functions (Tools)

Dissolve Buffering Near overlay

GIS ANALYSIS TOOLS

QUERIESAsk questions about GIS databases: Where are the older stands? Which roads are paved? Which trails are authorized? Which water sources are within a certain distance of a

road?

Note that the queries do not inherently result in a new layer… They usually only highlight features (which could be exported to a new layer afterwards)

QUERIESWhere are the thinnable stands?

Age 30 and Age 40 Age 30 and Age 40 and MBF 30

QUERIES

Structured Query Language (SQL) uses standard operators

e.g. = > < + - * “and” “or” “not”

standard order of operations add/subtract before multiply/divide use parentheses to “isolate” terms

QUERIESExample: select stands greater than 30 acres with grass

understories and a mean quadratic diameter less than 20 inches.

query for above:

(area > 30) and (understory = “grass”) and (QMD < 20)

QUERIES: Not always the Best

#Y#Y#Y

#Y #Y

Which water sources are within a certain distance of a road?

we need more information.

perhaps a new database layer.

“buffering” may help answer this question

Spatial Joins

• Joins attributes from one feature class to another based on a spatial relationship (INTERSECT, CONTAINS, WITHIN, CLOSEST)

– points in polygon (identifies polygon in which point is located)

– polygon in polygon (identifies polygon in which polygon is located)

– lines in polygon (identifies polygons crossed by line)

– points on lines (calculate distance to nearest line)

– points on points (calculate distance to “nearest neighbor” point)

operate on tables and normally creates a new table with additional variables, but does not modify spatial features themselves

Query Vs. Spatial Join

• Selection: simply selects (“highlights”) entire spatial features in the target layer, but doesn’t modify these features

– Selection only– Only Selected features (a subset of all features) are “output”– No new output file saved unless you use Export/data

• joins: operate on tables and normally creates a new table with additional fields or variables (columns), but again does not modify actual spatial features (rows)

– adds attributes (columns) to the layer’s table from another layer’s table– All features are “output”– No features modified– No new output file saved unless you use Export/data

Note: Different approaches can be used, in some cases, to produce same results.

BASIC STATISTICS

statistics can help determine meaning within the data

simple, sum, count, mean, maximum, range, variance and standard deviation

calculates statistics for a combination of fields, for example: by combining the ‘State’ name field &

‘Population’ fields, we can calculate the average state population

• Right Click on the Field• Go to “Summarize”

Choose statistic to summarize

BASIC STATISTICS: Calculating Fields

• New values can be calculated based on other attribute values

• Simple algebra and more advanced math operations

• Text functions for picking part of a string or modifying case

BASIC STATISTICS: Calculate Geometry

• Length, Area, or X,Y coordinates

• Choose coordinate system of source or data frame

• Pick units of measurement

• Make sure field has units in the name

Functions (Tools): Types of Analysis

Layer 1Function 1 Layer 2

Function 2 Layer 3

Function

Layer 2

Layer 1

Layer 3

Dissolve Tool

– Similar to the merge function in Editor toolbar

Dissolve Tool

• Input Features• Output Feature Class• Dissolve field(s)

(optional)• Statistics (optional)

• Create multipart features (optional)

Buffering

• Buffering creates a polygon using a specified distance from a point, line, or other polygon

Buffer Tool

• Input Features• Output Feature

Class• Distance

– Linear unit (pick units)

– Field (attribute table)

• Side/End type• Dissolve Type

Buffer application

• 500 ft. buffer applied to houses

• Buffer overlaps transfer station

Multiple-Ring Buffer

• Creates buffers for many distances at once

• Dissolve option makes them non-overlapping

Near Tool

• Calculates distance from input features to nearest feature in other layer(s)

Near Tool

• Output has field with ID and distance of feature

• Multiple near layers can be calculated at once

• Options to include:– Location (X,Y)– Angle (degrees)

Near Application

Overlay

• Intersect

• Identity

• Symmetrical Diff.

• Union

• Clipping/Erasing

Intersect (AND Operator)

• Polygons split at feature boundaries of both datasets

• Only overlapping areas are kept

• Attribute tables are updated in output

Identity

• Similar to intersect but input features are not clipped

• Features split along identity polygon edges

• Attribute tables are combined in output

Symmetrical Difference (XOR Operator)

• Simply differencing the two layers (overlapping areas are ignored)

Union (OR Operator)

• Like Intersect but both input and union features are retained

• Output features have attributes from both input and union layers

Clipping

• Using one layer like a cookie cutter for another

• Any vector can be used as input

• Clip feature must be a polygon

Erasing

• The opposite of clipping– Erase feature used to remove a portion of

the input data (point, line, or polygon)

Intersect / Identity / UnionWhich function is which?

Summary

Soil Type

Overlaid with

Crops Production(ton/ha)

Overlay Result

GIS Technology: Relationship between Land use and Crop Productivity

Overlay Analysis

Spatial Analysis Tips

• Using these tools in conjunction with each other can produce some useful data– Clip a polygon using a buffer– Calculate new fields to determine percentages

• Name your output files so you will remember the inputs and the tool you used– i.e. (LandUse_Buffer_Intersect.shp)

• When in doubt, check the help documentation

Homework & Lab

• HW 7: Ch. 6 p. 149 – 165, answer Q’s 1, 3, 4, 12

• Lab this week: Spatial (vector-raster) Analysis– The instructions will be given (Do NOT

bring your lab book on Wed)

• Next lecture would be on April 17th