Map Analysis with Raster Datasets

Francisco Olivera, Ph.D., P.E.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 map analysis with raster datasets, the digital spatial datasets consist of grids.

Spatial Analyst

To enable raster data analysis capabilities, the Spatial Analyst ArcView extension has to be loaded.

The Spatial Analyst adds the Analysis and Surface menus, as well as the Histogram button and Contour tool, to the ArcView GUI.

A significant number of additional capabilities are also available through Avenue scripts.

Grid Datasets

Grid Data Structure

Cellular-based data structure composed of square cells of equal size arranged in rows and columns that store the value of a terrain parameter.

Cells that do not store any value are assigned a NODATA code and are called NODATA cells.

Number of columns

Num

ber

of

row

s

Cell size

(x, y)

Grid Properties

Type: Integer or Floating Point. Depending on the type of numbers the grid cells store, a grid is considered Integer (if it stores integer values) or Floating Point (if it stores real values). Floating Point grids, storing the same information as an Integer grid, take significantly much more storage space.

Status: Permanent or Temporary. Many grids created in ArcView are stored as temporary files and are subject to be erased automatically. A temporary grid can be saved as a permanent grid by clicking on Theme/Save Dataset.

Value Attribute Table

Value Attribute Tables (VATs) are tables associated with grids which have two fields: Value and Count. The Value field lists all values found in the grid, and the Count field the number of cells that have that value.

Only Integer grids have VATs.

VATs are created automatically for Integer grids with less than 500 unique values or with a range of values that does not exceed 100,000. Otherwise, the VAT has to be created with Avenue requests.

Analysis Properties

Analysis Properties are defined for a View document.

Analysis Extent defines the rectangle in which the grid is contained. The Analysis Extent can be defined same as a specific grid or with coordinates.

Cell Size defines the length of the cell side. The Cell Size can be defined same as a specific grid or with a number.

It is fundamental to have all grids of a view aligned and with their cells coinciding exactly. Therefore, the Analysis Extent and Cell Size must be set before any grid is created in the View.

Analysis Properties

Analysis Properties are defined for a View document.

The Number of Rows and Number of Columns are self explanatory. These properties would be redundant if the Analysis Extent and Cell Size have been defined already.

The Analysis Mask flags the cells – within the Analysis Extent – where grid values are not calculated. Grid values are assigned NODATA wherever the mask has NODATA cells. The Analysis Mask is defined same as a specific grid.

Vector-to-Raster Conversion

Point-to-grid:

Each point is converted into the grid cell where it is located. The cell value is a user-selected attribute of the point dataset.

If two points coincide in the same cell, one is chosen randomly for the cell value. Coincidence of two or more points within a cell might reflect inconsistency between the resolution of the grid and point dataset.

To convert a point dataset into a grid, click on the Theme/Convert to Grid menu.

Vector-to-Raster Conversion

Line-to-grid:

Each line is converted into the grid cells with which it intersects. The cell value is a user-selected attribute of the line dataset.

If two lines coincide in the same cell, one is chosen randomly for the cell value.

To convert a line dataset into a grid, click on the Theme/Convert to Grid menu.

Vector-to-Raster Conversion

Polygon-to-grid:

Each polygon is converted into the grid cells whose center it contains (centroid method used by ArcView). The cell value is a user-selected attribute of the polygon dataset.

If two (overlapping) polygons coincide in the same cell, one is chosen randomly for the cell value.

To convert a polygon dataset into a grid, click on the Theme/Convert to Grid menu.

Raster-to-Vector Conversion

Grid-to-polygon:

All adjacent cells (i.e., share a side) with the same value are aggregated into a single polygon.

The cell value is stored in an attribute Grid Code of the polygon dataset.

Polygon outlines are smoothed to avoid "jagginess".

Grids have to be Integer grids.

To convert a grid into a polygon dataset, click on the Theme/Convert to Shapefile menu.

Grid-to-point and grid-to-line conversions are available in ArcView, but not in the standard interface.

Raster Functions

Raster functions create output grids using input grids as arguments.

Raster functions are classified into:

Local functions

Focal functions

Zonal functions

Global functions:

Local Functions

The value of an output grid cell depends on the value of the cells of the input grids that have the same location. Neighbor cells have no influence on the output values.

Local functions can have one or many input grids as arguments.

+

Focal Functions

The value of an output grid cell depends on the value of the cells of the input grids in the neighborhood.

The neighborhood can be defined in different ways.

Focal functions usually have one input grid as argument, but could have more than one.

Zonal Functions

The value of an output grid cell depends on the value of the cells of the input grids of the same zone.

A zones grid has to be one of the function arguments.

Besides the zones grid, zonal functions usually have only one more input grid as argument, but could have more than one.

Global Functions

The value of an output grid cell depends on the value of all the cells of the input grids.

Global functions usually have one input grid as argument, but could have more than one.