Vector & Raster Data

Union Method

*Keeps all areas of the two input layers in the output.* 1) Preserves all features from inputs (polygon & polygon only) -- Output combines area extents of both inputs 2. Attribute values are assigned by containment 3. Attribute inheritance

Intersect Method

*Preserves only the area common to the two input layers in the output.* 1) Preserves features that fall within the area extent common to both inputs (overlap area) 2) Attributes from both layers exist 3) Order of input and overlay does *NOT* matter 4) Polygon on polygon, line, or point

Buffering Applications: Examples

*Protection & Planning/ Regulatory Purpose* - City ordinance - no liquor stores w/in 1000 ft of school - Gov. - 2mi buffer by streams to reduce sedimentation - Forestry - no oil & gas drilling w/in 500 ft of roads *Exclusion* - Police: Protesters 300 ft. from a building - Conflict resolution: Demilitarized zone between North & South Korea *Inclusion* - Industrial park may be w/in 1mi of a heavy-duty road

Minority

*The least frequently occurring value in each zone is assigned to all cells in that zone.* When there is a tie for the minority value in a zone, the output for all cell locations in the zone is assigned the lowest of the tied values.

Majority

*The most frequently occurring value in each zone is assigned to all cells in that zone.* *The highest value that occurs the most is assigned to the output cell in that zone.* When there is a tie for the majority value in a zone, the output for all cell locations in the zone is assigned the lowest of the tied values.

Slivers

- Common error from overlaying - *Very small polygons along correlated or shared boundary lines of the input layers.*

What are local operations in raster analysis?

- Local operations are *cell-by-cell operations.* - A local operation can create a new raster from either a *single input raster or multiple input rasters.*

How do we run a vector data analysis?

- Use the spatial features of point, line, and polygon as *inputs* - *Topology* can also be a factor for some vector data analyses such as buffering and overlay.

Raster data analysis is based on ____ and ____ . T/F: Raster data analysis can be performed at the level of individual cells, or groups of cells, or cells within an entire raster.

- cells and rasters - True

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Raster Data How is it represented? What is it used for?

-Array of cells in rows and columns used to represent objects -Useful as background maps and for spatial analysis

Vector Data How is it represented? What is it used for?

-Point, line and polygon representations -Widely used in cartography, and network analysis

Neighborhood Statistics

1) *Summary statistics*, such as maximum, minimum, range, sum, mean, median, and standard deviation, 2) *Tabulation of measures*, such as majority, minority, and variety.

Buffering Applications: Buffering can be used . . .

1) As protection zones 2) For planning or regulatory purposes 3) Exclusion 4) Inclusion

How does an overlay operation work?

1) It combines the geometries and attributes of two or more feature layers to create the output. 2) The geometry of the output represents the geometric intersection of features from the input layers.

What are the two major types of spatial data models?

1) vector 2) raster

What are some tools used in GIS to manipulate and manage features? (x8)

1. Dissolve 2. Clip 3. Append 4. Select 5. Eliminate 6. Update 7. Erase 8. Split

Circle Operation

A circle, shown in white, is used to extract cell values from the input raster (a). The output (b) has the same area extent as the input raster but has no data outside the circular area. See handout!

What is a data model?

A mathematical construct for representing real-world objects as data

Neighborhood Operations

A neighborhood operation involves a *focal cell and a set of its surrounding cells.* The surrounding cells are chosen for their *distance and/or directional relationship* to the focal cell.

*Areal Interpolation* - An application of Overlay

A process of transferring known data from one set of polygons (source polygons) to another (target polygons) - Helps solve the areal interpolation problems.

Point-in-Polygon Operation

A spatial operation in which points from one feature dataset are overlaid on the polygons of another to determine which points are contained within the polygons.

Straightline Distance

A straight-line distance is measured from a cell center to another cell center.

Zonal Operations

A zonal operation works with groups of cells of same values or like features. These groups are called *zones.* Zones may be contiguous or noncontiguous

All overlay methods are based on the Boolean connectors of . . .? (x3)

AND, OR, and XOR.

What does the accuracy of analysis depend on?

Accuracy of analysis depends on *accuracy of spatial features in terms of location and shape.*

Operations for *raster data generalization* include ___ and ___ .

Aggregate and RegionGroup.

Allocation

Allocation produces a raster in which the cell value corresponds to the closest source cell for the cell.

Analysis Mask Operation

An analysis mask (b) is used to clip an input raster (a). The output raster is (c), which has the same area extent as the analysis mask.

Append Tool

Append pieces together two adjacent layers into a single layer but does not remove the shared boundary between the layers See Handout!!

How do we remove slivers?

ArcGIS uses the *cluster tolerance*, which forces points and lines to be snapped together if they fall within the specified distance.

Buffering

Based on the concept of proximity, buffering creates two areas: 1) one area that is within a specified distance of select features and 2) the other area that is beyond.

Operations for *raster data management* include ____ and ____ .

Clip and Mosaic.

Continuous Distance

Continuous distance measures from a stream network.

How is raster "field-based"? What does that mean? Give examples.

Describing the earth as a collection of spatial distributions of phenomena Ex. elevation or temperature

How is vector "object-based"? What does that mean? Give examples.

Describing the earth as a space that is filled with discrete, identifiable units (in other words, objects) that have some sort of spatial reference, usually in the form of geographic coordinates. Ex. houses, roads, rivers, etc. Objects can be counted!

What are examples of when raster data models would be used?

Digital elevation data, digital orthophotos, scanned maps, graphic files

Direction

Direction produces a raster in which the cell value corresponds to the direction in degrees that the cell is from the closest source cell.

Eliminate Tool

Eliminate removes some small slivers along the top boundary (A). See Handout!!

Erase Tool

Erase removes features from the input layer that fall within the area extent of the erase layer. •"Cookie cutter" •Only input layer features and attributes exist in output • *Order of input and overlay matters* •Polygon on polygon, line, or point

What is *error propagation* in overlay? What is an example of error propagation?

Error propagation results from *inaccuracies of the input layers.* *Slivers* are examples of errors in the inputs that can propagate to the analysis output.

Local Operations: Single Raster How does it work?

Given a single raster as the input, a local operation computes each cell value in the output raster as a mathematical function of the cell value in the input raster. *Ex. Converting a slope raster from percent to degrees.*

An overlay operation is called ____ or ____ if it uses the following expression: [(input layer) AND (identity layer)] OR (input layer).

Identity or Minus

An overlay operation is called _____ if it uses the *AND* connector.

Intersect

Local Operations: Multiple Rasters How does it work?

Local operations with multiple rasters are the equivalent of vector-based overlay operations. Besides mathematical functions that can be used on individual rasters, other measures that are based on the cell values or their frequencies in the input rasters can also be derived and stored on the output raster of a local operation with multiple rasters. *Ex. Taking the mean of three rasters (cell by cell) and putting them in one output raster*

Map Algebra

Map algebra refers to an informal language with syntax similar to algebra, which can be used to facilitate manipulation and analysis of raster data.

How does map algebra work?

Map algebra uses an expression to link the input and the output; besides the input and output, the expression can be composed of GIS tools, mathematical operators, and constants.

When buffering we MUST know the _____ _____ . Why is this important?

Measurement unit (feet, meters, kilometers) Distance measurements are used in buffering, thus, *positional accuracy* of spatial features determines *accuracy of buffer zones*

Topological overlay: how it works

Overlay operations can take polygon, line, or point layers as the inputs, and creates an output of a *lower-dimension feature type*

Symmetrical Difference or Difference

Preserves only the area common to only one of the input layers in the output.

Map Overlay

Process of taking two different thematic maps of the same area and overlaying them one on top of the other to form a new map layer. (Sometimes, overlay is performed for visualizing spatial relationship of two maps, thus no new map is created.)

Identity Method

Produces an output that has the same extent as the input layer. But the output includes the geometry and attribute data from the identity layer. *LOOK AT HANDOUT*

Select Tool

Select creates a new layer (b) with selected features from the input layer (a). See Handout!!

Notes on Raster Data Analysis

Some raster data operations use a single raster, while others use two or more rasters. Raster data analysis is also related to the type of cell value (numeric or categorical values) in the input raster(s).

Split Tool

Split uses the geometry of the split layer to divide the input layer into four separate layers.

An overlay operation that uses the XOR connector is called _____ _____ or _____.

Symmetrical Difference or Difference.

T/F: A zonal operation may work with a single raster or two rasters

TRUE

True/ False: Feature layers to be overlaid must be of *same* coordinate system.

TRUE Note: - For UTM, features must be in same zone (e.g. UTM Zone 15) and have same datum (e.g. WGS 84)

What is the Raster Analysis Environment?

The analysis environment refers to the area for analysis and the output cell size.

Polygon-on-Polygon Operation

The output combines the polygon boundaries from the input and overlay layers to create a new set of polygons.

Line-in-Polygon Operation

The output contains the same line features as in the input layer but each line feature is dissected by the polygon boundaries on the overlay layer.

Physical Distance Measure Operations

The physical distance measures the *straight-line or Euclidean distance.* Physical distance measure operations *calculate straight-line distances away from cells designated as the source cells.*

Topology

The study of how items are related to one another in space; a configuration.

An overlay operation is called _____ if it uses the OR connector.

Union

Update Tool

Update replaces the input layer with the update layer and its features.

How do vector models represent objects?

Uses points and their *x, y coordinates* to represent spatial features *Discrete objects* are formed by *connecting line segments* starting and ending with points

In raster data models, what do the changes in cell values reflect?

Variation of phenomenon

What is the expression for Identity or Minus overlay operations?

[(input layer) AND (identity layer)] OR (input layer)

Raster Data Model

a way of representing spatial data that utilizes a series of equally spaced and sized grid cells

Given two rasters in a zonal operation - one input raster and one zonal raster - . . .

a zonal operation produces an output raster, which summarizes the cell values in the input raster for each zone in the zonal raster. See handout!!

The buffer area that is within the specified distance of select is called the ______ _______

buffer zone

Raster is _____ -based Vector is _____ -based

field object

Common neighborhoods include . . .

rectangles, circles, annuluses, and wedges. See handout!!

Operations for *raster data extraction* include . . .

use of a data set, a graphic object, or a query expression to create a new raster by extracting data from an existing raster.

Given a single input raster, . . .

zonal operations measure the geometry of each zone in the raster, such as area, perimeter, thickness, and centroid.

Clip Tool

• "Cookie cutter" • Creates an output that contains only those features of the input layer that fall within the area extent of the clip layer. •Polygon on polygon, line, or point

Identity Method Continued

•Preserves features that fall within the area extent of the input layer •Attributes from both layers exist where there is overlap •Order of input and overlay (identity) matters •A polygon on polygon, line, or point but *identity layer must be a polygon*

Dissolve Tool

•Removes boundaries between polygons or nodes between arcs •Features with same attributes are dissolved