Spatial Analysis using Vector Data

# 1Francisco Olivera, Ph.D., P.E.

Srikanth Koka Department of Civil Engineering

Texas A&M University

Map Analysis

Map analysis consists of inferring information – necessary for a given engineering/scientific task – from general information contained in digital spatial datasets.

In this case, the map analysis will use vector data (i.e., features such as points, lines and polygons).

Sample DatasetFive feature classes are used to explain the analysis tools:

A: polygon datasetB: polygon dataset C: line dataset D: point datasetE: point dataset

Sample DatasetIn addition to geometric information (area, perimeter and length), the feature classes have the following fields:

AConc: Mass of a certain fertilizer per unit area (kg/m2) (I)ATrees: Number of trees (E)BOwner: Land owner (I)CName: Road name (I)DRain: Annual precipitation depth (mm) (I)ERain: Annual precipitation depth (mm) (I)

GeoProcessing Wizard

The GeoProcessing Wizard extension allows one to modify and/or create new features classes.

Load the wizard by clicking on the Tools/GeoProcessing Wizard menu.

GeoProcessing - Dissolve

Creates a new feature class in which all features that have the same value for a given attribute are aggregated into a single record

Applies to one dataset only

User has the option of defining how attributes are recalculated

GeoProcessing - Dissolve

Dissolving B so that all land that belongs to the same person is represented by a single polygon.

Select the dataset you want to dissolve, select the key attribute which will be used to aggregate the features, and enter the path and name of the new feature class.

Select how the attributes shall be calculated (more than one option is allowed). The key attribute is copied by default.

GeoProcessing - Dissolve

After dissolving the feature class, boundaries between adjacent polygons that are aggregated disappear.

If aggregated polygons are not next to each other the effect of dissolving will not be obvious in the map, but in the table.

The figure shows only one record selected. The two polygons were two different records before dissolving.

GeoProcessing - Dissolve

After dissolving the dataset, the key attribute (i.e., BOwner) is copied, and two new attributes are created:

Count_BOWNER: number of polygons aggregatedSum_Area: sum of the previous attribute Area (as requested in previous window)

GeoProcessing - MergeCreates a new feature class from features of two or more feature classes.

Attributes of only one of the feature classes are retained. Attributes of the other feature classes are retained only if they have the same name.

The user has the option of selecting the dataset whose attributes will be retained.

GeoProcessing - Merge

Merging D and E so that all precipitation stations are in a single feature class

Select the feature classes you want to merge, select the dataset whose attributes will be retained and enter the path and name of the new feature class.

GeoProcessing - Merge

After merging the feature classes, all points are features of a single feature class.

GeoProcessing - MergeAfter merging the feature classes, the resulting feature class attribute table has the attributes RecNo and DRain because those are the attributes of D (the feature class whose attributes are retained).

Attribute ERain of E is not transferred, while RecNo is transferred. If Recno refers to the record number, it would be advisable to update it.

GeoProcessing - Clip

Creates a new feature class by clipping an input feature class with another clipping polygon feature class. The attributes of the input feature class are not altered (including the geometry).

Applies to two feature classes

GeoProcessing - ClipClip out C with the selected polygons of BDisslv to identify the road segments that are in Kelly's, Mary's, and Melissa's property.

Select the feature class you want to clip, select the polygon feature class you want to use to clip and enter the path and name of the new feature class. Check the Use Selected Features Only boxes if it applies.

GeoProcessing - ClipAfter clipping out the C feature class, only those segments within the selected polygons of BDisslv are considered.

Note that features in the resulting feature class are obtained by intersection of the input and the outline of the clipping feature class.

GeoProcessing - ClipAfter clipping, the attributes of the input feature class are copied but not updated. The length, area and perimeter need to be updated (i.e., recalculated.)

GeoProcessing - Intersect

Creates a new feature class in which the features are the result of intersecting the features of an input feature class with the polygons of an overlay feature class.

Resulting features have attributes from both feature classes.

Applies to two datasets.

GeoProcessing - Intersect

Intersecting A (input) and B (overlay) so that for each resulting polygon the mass of fertilizer per unit area and the owner are a unique combination of values.

Select the input and overlay feature classes you want to intersect and enter the path and name of the new feature class. Check the Use selected features only boxes if it applies.

GeoProcessing - Intersect

After intersecting the feature classes, the number of polygons significantly increases, and the resulting feature class includes attribute data from both sources.

GeoProcessing - Intersect

After intersecting, all attributes from both datasets are copied.

The attributes Length, Area and Perimeter correspond to the original feature classes and have to be recalculated.

GeoProcessing - UnionCreates a new feature class by combining the polygons of an input feature class with the polygons of an overlay feature class. The resulting dataset has the extent of both.

Applies to two polygon feature classes.

Union is very similar to Intersect, but in Union the parts of the polygons that do not intersect have zero for the non-defined attributes.

GeoProcessing - UnionUnion of A (input) and B (overlay) so that for each resulting polygon the mass of fertilizer per unit area and the owner are a unique combination of values.

Select the input feature class, select the overlay feature class, and enter the path and name of the new feature class. Check the Use Selected Features Only boxes if applies.

GeoProcessing - Union

After "unioning" the feature classes, the number of polygons significantly increases and the resulting dataset includes attribute data from both sources.

GeoProcessing - UnionAfter "unioning," all attributes from both datasets are copied. Polygons that correspond to only one dataset get zero or blank for the attributes of the other dataset.

The attributes Length, Area and Perimeter correspond to the original datasets, and have to be recalculated.