gis

MODULE 6 MAP OVERLAY AND GEOPROCESSING

Geoprocessing

Map Overlay

Overlay with Attributes

Other Spatial Analysis Functions

Map Overlay in ArcGIS

Geoprocessing and Map Overlay

Geoprocessing , as discussed in a previous

module

--involves stringing together two or more functions to

solve a specific spatial problem

Map Overlay

Combines two layers to create a new output

feature class

--contains information from both of the inputs

Overlay functions fall into two categories

--those that do not combine attributes and those that do

Overlay Functions That Do Not Combine

Attributes

Covered in Module 4 (MGIS chapter 8)

1) Clip

2) Erase

Overlay Functions That Combine Attributes

1) Intersect

2) Union

Clip Input Output

Intersect

Union

OVERLAY WITH ATTRIBUTES

Vector overlay relies on geometry and topology:

Layers being overlaid must be topologically

correct --lines meet at nodes and all polygon boundaries are

closed

--this is why Quality Assurance is very important

--to correct for such errors before you get to this point

The topology of the new layer that is produced: --calculated from the intersections of lines and polygons

from the input layers using geometry

--this is a computation-intensive process

Overlay with Attributes

Intersect and Union Functions

--similar to spatial joins

--correlate features based on their spatial relationships to one

another

How they differ:

--spatial joins fail when spatial features do not overlap exactly

OVERLAY WITH ATTRIBUTES:

UNION

Example: A road crosses three landuse types --how would a land-use type be assigned --completely within option returns no land-use class,

since it does not lie entirely within a polygon --summary would not work, since nominal data types

cannot be averaged --ideally, you would split the road into three sections

and assign the land use to each

Overlay functions can do this --enforces a one-to-one relationship between features --now each road segment can be assigned one land use

value

Union

Combines two polygon layers

--keeps all of the areas

--merges the attributes for both layers

Creates all possible polygons from the combination of features in both layers

--produces a new feature class with new polygons

--each possesses the original attributes of its parents

Only polygon layers can be used with a Union

Union

Splitting a road to assign attributes to each piece

Each new road segment retains its original attributes

and receives the land use code and other attributes of

the polygon within which it falls

OVERLAY WITH ATTRIBUTES: INTERSECT

Intersect

Only keeps those portions of features that are

shared by both input layers --allows user to find out where two or more conditions

hold simultaneously

The Intersect Command Is More Versatile than

the Union Command --both input and overlay layers may contain points,

lines, or polygons

--the output geometry cannot exceed the

dimensionality of the lowest dimension input

--polygon-on-polygon = points, lines or polygons

--line-on-line = lines or points

--point-on-point = points

Intersect

OTHER ANALYSIS FUNCTIONS: DISSOLVE

Overlay is one type of spatial analysis available with GIS

Other commonly used functions: --dissolving --buffering --appending --merging

Dissolve Function Used to group features that share the same value for a particular attribute --road segments that share the same street name can be

dissolved into one continuous street --ponderosa pine stands of different ages can be dissolved

on cover_type field to create polygons that contain ponderosa pines

This can be thought as a generalization process

When features are dissolved --the output layer is a new file --contains the single attribute on which the dissolve was

based --the user can specify additional fields to summarize the

information from the original features --average tree age for each output polygon

--based on averaging tree ages of the polygons before the dissolve

OTHER ANALYSIS FUNCTIONS: BUFFER

Buffering—results in the creation of a “zone of interest” around a feature, or set of

features

Questions that Could be Answered Using a

Buffer

1. Which hotels are within 200, and 400,

meters of a Main road

a. Create a 200-meter wide, then a 400-meter

wide, buffer around the road [(b) in the above

image]

b. Find out which hotels fall within this buffer

zone using a point-in-polygon overlay

c.Use a query to find the names of the hotels

that lie within the buffer zone

2. Which areas of a city lie within 5, and

within 10, miles of a cell phone tower:

create a multi-part buffer

3. Creation of a 20-mile, and a 40-mile,

buffer around a nature preserve in the

Amazon Basin

OTHER ANALYSIS FUNCTIONS: APPEND AND MERGE

Append

Combines the features of two or more layers to create a single existing layer --the appended layers must have the same feature type as the

target (i.e. both polygons, both lines, or both points)

--the input layers must also share the same coordinate system

--the layers can overlap

In order to combine the attribute information from both layers --the attribute fields must have the same definition and

must occur in the same order in both tables --if the two tables differ, one can use the NOTEST

option

--if a field in input layer has same name and data type as the target layer, the information will be copied into the target

--fields w/o matching names will not be carried into the target

Merge

Similar to append --offers a more flexible treatment of attribute tables

--does not insist that the two tables match

--allows the user to specify the fields to be included in the output feature class

--and which table they will come from.

GEOPROCESSING IN ARCGIS

GIS analysis involves various functions that operate

on data objects --geoprocessing applies one or more of these functions in

sequence to solve a problem or investigate the

properties of data sets

Geoprocessing tools may executed in various ways --toolbars and menus provide interactive control of these

tools

--menus and toolbars can be customized to add frequently

used toolbars

--users can also create their own tools and add them to the

toolbar

ArcToolbox Organizes all of the installed tools into one central location

--provides a window with nost of ArcGIS’ functionality

--tools can be run from either ArcMap or ArcCatalog

Each tool has a set of parameters (inputs) that must

be specified before the tool is run --some are required

--others are optional

Users can also type in commands --may be faster than filling boxes in a window for more

experienced users

MODEL BUILDER

Model Builder

Provides a graphic framework for stringing tools

together and executing them in sequence

1) The graphic records the steps and parameters used

to execute an analysis

--records the methodology for future reference

2) Models can be used to explore the consequences

of different parameters on the final outcome --simplifies process of analysis when it needs to be

repeated on a regular basis for a time sequence, for

example

Models can be shared with other users --an expert can set up a procedure to be used by less

experienced users

Models can incorporate --decision structures

--conditional statements

--iteration functions

Models can also be saved as scripts to provide a

starting point for creating geoprocessing programs --scripts are programs that contain conditional statements

(if/then), iterative loops, and other control structures

--permit sophisticated analysis

Input parameter (x,y tolerance) added

to a model in Model Builder

GEOPROCESSING ENVIRONMENTS

Environment Settings set by the user can affect

the operation of tools and commands in ArcGIS

Example:

The default setting for the coordinate system

setting is Same as Input --means that the output file has the same coordinate

system as the input file

--user can instead specify a specific coordinate system

(UTM Zone 13 NAD 1983, for instance)

--every output feature class would have the UTM

coordinate system regardless of the input system

Application settings are saved along with a map

document --it is thus possible for the settings to be operating

without the user’s knowledge

--it is recommended to check environment settings if

one is getting unexpected results from the tools

When a map document is opened, all settings return to

their defaults

--settings made in ArcCatalog will become the default

for all new map documents, however

COORDINATE SYSTEMS

Tools accept any coordinate system or

combination of coordinate systems for input --rules of precedence dictate coordinate system of

the output

1) If output placed in a feature dataset --the coordinate system will always match that

of the feature dataset

2) If coordinate system is set in the

Environment settings and the output will

not become part of a feature dataset

--the Environment settings coordinate system is

used

3) If the Environment setting is not set --the default rule applies

--i.e. the output will match the coordinate

system of the first input to the tool

FEATURE AREAS AND LENGTHS

Geodatabases and coverages automatically create

and update fields containing the areas and

perimeters of polygons or the lengths of lines --shapefiles do not maintain this info

Users can create AREA or LENGTH fields for

shapefiles and calculate the values manually

Care must be taken --if polygons in a shapefile are clipped, dissolved, or

intersected, or undergo any other operation that changes

their shape

--AREA fields will not be updated automatically

--the user must manually update the fields again to ensure

they are correct

Also: --be cautious using an AREA, PERIMETER, or LENGTH

field in a shapefile

--the Calculate Geometry tool can be used for updating

areas, lengths, and perimeters