Basics of Raster Data

DOQ

digital orthoimage quadrangle

cell value

A characteristic of raster data that represents phenomena (for example, elevation or soil type) illustrated by the raster. Cell value can be positive or negative, an integer (representing discrete data), or a floating point (representing continuous surfaces). Cell data can also have a NoData value, representing an absence of data.

mosaic dataset

A data model for efficiently storing and managing raster data. Mosaic datasets allow you to store, manage, view, and query small to vast collections of raster and image data. They manage and visualize data, and also serve as a tool for disseminating imagery.

surface

A geographic phenomenon represented as a set of continuous data (such as elevation); a spatial distribution which associates a single value with each position in a plane, usually associated with continuous attributes.

picture

A graphic image. Examples of pictures include scanned maps, drawings, and building photographs.

scanned map

A hard-copy map or image that has been captured in digital format using a device called a scanner.

aerial photograph

A photograph of the earth's surface taken from a platform flying above the surface (usually an aircraft). Aerial photography is often used as a cartographic data source for basemapping, locating geographic features, and interpreting environmental conditions.

Lidar image

A profile slice of a lidar all-return point cloud. Each return is labeled distinctly. This basic lidar concept can be used to illustrate the fact that one pulse out of a lidar sensor can have up to four associated returns. This graphic illustrates an active sensor that provides its own energy source for illumination and includes the ability to obtain measurements any time, regardless of the time of day or season.

digital raster graphic (DRG)

A raster image of a scanned U.S. Geological Survey (USGS) standard series topographic map, usually including the original border information, referred to as the map collar, map surround, or marginalia. Source maps are georeferenced to the surface of the earth, fit to the universal transverse Mercator (UTM) projection, and scanned at a minimum resolution of 250 dpi. The accuracy and datum of a DRG match the accuracy and datum of the source map.

digital elevation model

A representation of continuous elevation values over a topographic surface by a regular array of z-values, referenced to a common datum. It is typically used to represent terrain relief.

discrete thematic raster

A representation of objects that have known and definable boundaries, such as buildings, wells, land parcels, and lakes. A lake, for example, is a discrete object within the surrounding landscape—the location at which the water's edge meets the land can be definitively established.

continuous raster

A representation of phenomena in which each location on the surface is a measure of the concentration level or its relationship from a fixed point in space (for example, elevation and aspect) or from an emitting source. A continuous raster can also include phenomena that progressively vary as they move across a surface from a source (for example, fluid and air movement).

image

A representation or description of a scene, typically produced by an optical or electronic device, such as a camera or a scanning radiometer. Common examples include remotely sensed data (for example, satellite data), scanned data, and photographs.

Surface map created using GPS data

A surface map created using GPS data. Capturing GPS data is manually intensive but can also provide highly accurate locational data. This data can also be used to verify or correct the accuracy of other digital data sets.

Identify ideal raster dataset for a scenario Performing a land-use study

A thematic or a discrete raster groups the values of multispectral data into classes and assigns a categorical value. These rasters are ideal for land-use studies because each of the pixels belongs to a distinct class. Land-use maps provide information about zoning (marking out areas designated for specific types of land use so that people developing land know which kinds of uses will be allowed), utilization (farming, waste storage, residential use), and so forth.

Identify ideal raster dataset for a scenario Distinguishing land-water edges

A thematic raster (for example, a classified image) has well-defined boundaries between different land forms and land use; it is easy to define precisely where an object begins and where it ends.

Raster or vector dataset? Purpose: Identify museum locations in your area

Addresses are geocoded as point locations and are represented as vector points. Typically, retail stores, customers, crime locations and incident patterns, disease cases, businesses, etc., are represented as vector points.

orthophoto

An aerial photograph from which distortions owing to camera tilt and ground relief have been removed. An orthophoto has the same scale throughout and can be used as a map.

sensor

An electronic device for detecting energy, whether emitted or radiated, and converting it into a signal that can be recorded and displayed as numbers or as an image.

Radar image

An image of Chattanooga, Tennessee, obtained using the Side-Looking Airborne Radar image-producing system. Radar uses relative long wavelengths that allow these systems to "see" through clouds, smoke, and some vegetation. Radar images are currently being used for studies of ecology, hydrology, oceanography, geology, ice sheets, glaciers, icebergs, and archeological exploration.

Aerial image

An image of a portion of the surface of the earth taken from a platform flying above the surface but not in orbit, usually an aircraft. An example of a passive sensor that can only be used to detect energy when the naturally occurring energy is available. Aerial imagery is often used as a cartographic data source for basemapping, locating geographic features, and interpreting environmental conditions. For example, foresters use aerial images for preparing forest cover maps, locating possible access roads, and measuring quantities of trees harvested.

Thermal image

An overlay of a USGS photograph and a thermal image of the same area, highlighting several active breakouts on the coastal plain of the Puhi-o-Kalaikini Ocean Entry. Thermal infrared radiation refers to electromagnetic waves with a wavelength of between 3.5 and 20 micrometers.

Which of the following should be considered when specifying the cell size in raster data?

Application and analysis to be performed

You would like to classify a land-cover raster dataset based on its vulnerability to wild fires. The land-cover values represent different types of vegetation, except two that represent water and barren land. If you would like to order the types using a scale of 1 to 9, where 1 indicates not vulnerable and 9 indicates very vulnerable, which of the following methods could you use to exclude areas of water and barren land from your analysis?

Assign the values representing water and barren land a new value of NoData.

remote sensing

Collecting and interpreting information about the environment and the surface of the earth from a distance, primarily by sensing radiation that is naturally emitted or reflected by the earth's surface or from the atmosphere, or by sensing signals transmitted from a device and reflected back to it. Examples of remote-sensing methods include aerial photography, radar, and satellite imaging.

You have a raster that contains temperature data. The cell values range from 1 to 9, representing 5-degree increments. Is this raster discrete or continuous?

Continuous

multispectral data

Data that is captured at specific frequencies across the electromagnetic spectrum. Multispectral data allows for the extraction of additional information not seen by the human eye.

field-based

Describing the earth as a collection of spatial distributions of phenomena (for example, elevation or temperature).

object-based

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. Objects that might be found in this space include houses, factories, roads, rivers, and lakes. One characteristic of the discrete object view is that objects can be counted (for example, 47 houses in a particular subdivision).

DEM format

Digital Elevation Model

Identify ideal raster dataset for a scenario Showing topographic relief displacement

Digital elevation models store the topographic elevation of a surface as a single value in each pixel, which can be used for computations and deriving other products like hillshade, which can show relief displacement using color gradation.

IMG format

ERDAS IMAGINE

GIF format

Graphics Interchange Format

polyline

In ArcGIS software, a shape defined by one or more paths, in which a path is a series of connected segments. If a polyline has more than one path (a multipart polyline), the paths may either branch or be discontinuous.

data model

In GIS, a mathematical construct for representing geographic objects or surfaces as data.

JPEG format

Joint Photographic Experts Group

In the ArcMap table of contents, which of the following would you use to determine the data source of a layer?

List By Source button

MrSID format

Multi-resolution Seamless Image Database

Match the rasters with their corresponding bands Landsat image

Multiband raster Multiband rasters represent different wavelengths of energy from along the electromagnetic spectrum. Landsat imagery is data collected from seven different bands of the electromagnetic spectrum. All seven bands represent data from the visible, near-infrared, and mid-infrared regions. In addition, band 6 collects data from the thermal-infrared region.

NITF format

National Imagery Transmission Format

What does the size of the raster indicate?

Number of rows and columns

Identify ideal raster dataset for a scenario Creating basemaps for field use

Orthophotos are aerial photographs rectified for relief errors. They are best suited for serving as field-use basemaps.

Raster or vector dataset? Purpose: Identify areas of poor plant growth on a farm

Raster data is often used in agricultural management. Using satellite or aerial imagery, farmers can identify areas for improving land-management practices.

Raster or vector dataset? Purpose: Model water flow over land based on surface elevation

Rasters are ideally suited for representing data that changes continuously across a surface. Rasters can be used in surface hydrology to model water flow over a surface, to determine the water's origin and destination, to describe the behavior of water as it moves across a surface, to pinpoint the location of watersheds and stream networks, or to track runoff from land to stream.

Raster or vector dataset? Purpose: Analyze tornado damage

Satellite images are ideal because they provide an optimal level of detail for evaluating damage, both visually and algorithmically.

Match the rasters with their corresponding bands Black-and-white aerial photograph

Single-band raster An aerial image is a single-band raster taken from a platform flying above the earth. It is often used as a cartographic data source for basemapping, locating geographic features, and interpreting environmental conditions. All the information necessary to render an appropriate single-band raster is contained in a single grid of pixels.

Match the rasters with their corresponding bands Grayscale image

Single-band raster Grayscale images are digital representations of the grayness in each cell of a raster.

Match the rasters with their corresponding bands Elevation data

Single-band raster Typically, elevation is measured as a single value for a given location. Each cell contains only one value representing surface elevation.

TIFF format

Tagged Image File Format

Lidar-derived DEM

The graphic shows a bare-earth lidar-derived DEM, where all man-made structures and vegetation have been removed. This DEM is obtained using the point cloud lidar data, and is an example of a raster derived from vector data (lidar points). These DEMs have a wide range of uses such as earthquake hazard studies, hydrologic modeling, forestry, flood plain mapping, wetland studies, geologic studies, and a variety of analytical and cartographic projects.

Hillshade

The graphic shows a hillshade surface obtained by determining illumination values for each cell in a raster. This is done by setting a position for a hypothetical light source and calculating the illumination values of each cell in relation to neighboring cells. A hillshade can greatly enhance the visualization of a surface for analysis or graphical display, especially when using transparency.

Mosaic dataset

The graphic shows a mosaic dataset created using multiple rasters within a geodatabase. A mosaic dataset allows you to store, manage, view, and query collections of raster and image data.

Scanned Map derived from USGS topographic map

The graphic shows a scanned map. The map, photograph, transparency, or other object to be scanned is typically placed on a flat surface, and the device scans the image from side to side to record it. Scanning converts the image from analog to digital format.

Suitability raster

The graphic shows multiple rasters with values representing vegetation, precipitation, and elevation overlaid, and a series of operations applied to them to identify suitable areas for crop production. Suitability analysis is typically performed for land-use analysis, retail site selection, crime analysis, and so forth.

What is the bit depth of an image?

The number of binary integers that can be stored for each pixel

scanning

The process of capturing data from hard-copy maps or images in digital format using a device called a scanner.

resampling

The process of interpolating new cell values when transforming rasters to a new coordinate space or cell size.

cell

The smallest unit of information in raster data, usually square in shape. In a map or GIS dataset, each cell represents a portion of the earth and usually has an attribute value associated with it.

The following factors should be considered when specifying cell size:

The spatial resolution of the input data and the storage size of the raster The application and analysis to be performed The level of detail you want for the analysis to be performed Accuracy and precision and the desired response time

You have two 1:24,000-scale USGS topographic maps for the same area. One map was published in 1947; the other in 1987. You have traced the roads on both maps, and superimposed the tracings. The roads that appear on both maps do not align properly. What is likely to have gone wrong?

The two maps are based on different map projections.

Flow accumulation grid

This graphic shows a flow accumulation grid created using a lidar-derived DEM. This model is typically useful for forecast flooding, to generate drainage networks, and to order streams. A flow accumulation grid connects the direction of flow from cell to cell and determines the number of cells accumulating within a downslope flow path.

Lidar-derived intensity map

This graphic shows a lidar-derived intensity image created using a point-cloud lidar dataset (vector data). Intensity is a measure, collected for every point, of the return strength of the laser pulse that generated the point. The intensity images have a variety of uses in GIS applications such as feature detection and extraction, lidar point classification, and as a substitute for aerial imagery when none is available. If your lidar points include intensity values, you can make images from them that look something like black and white aerial photos, as shown in this graphic.

Thematic map

This graphic shows a raster representing thematic data derived using a satellite image classified by land-cover categories. A discrete thematic raster represents objects that have known and definable boundaries, such as buildings and wells.

Slope

This graphic shows a slope map calculated using the maximum rate of change in elevation over the distance between the cell and its eight neighbors. The vertical angle of the surface or local steepness of terrain is the slope. The lower the slope, the flatter the terrain; the higher the slope value, the steeper the terrain.

Why would you store data as a raster?

To represent continuous surfaces

USGS format

United States Geological Survey

You have a map document open with two layers, an orthophoto and a vector line data. You would like to visualize the two layers simultaneously in order to explore the data on the map. What do you do to accomplish this?

Use the Adjust Layer Transparency tool from the Effects toolbar

Raster or vector dataset? Purpose: Represent buildings

Vector polygons can be used to represent discrete geographical features such as lakes, tax parcels, counties, and buildings.

Raster or vector dataset? Purpose: Represent a railroad

Vector polylines can be used to represent linear features with explicit topology such as trails, rivers, roads, railroads, and topographic lines.

Can you find a DRG's spatial reference information by examining it in ArcMap?

Yes, by examining the Source tab in the Layer Properties dialog box.

Raster formats

formats each have different properties and requirements, and support specialized uses. Some offer high data compression, some handle color better than others, and some are designed specifically to store geographic data.

band

refers to the set of wavelengths on the electromagnetic spectrum. It is represented by a matrix of cell values that characterize the raster dataset—for example, spectral value (overall intensity or strength of light)

How many unique reflectance values can be stored in an 8-bit image?

256

ESRI Grid format

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How many colors can a DRG have?

13

Data compiled at which of the following scales is likely to be the most detailed?

1:1,000