Raster Based Midterm Review – Fall 16’
1. Understanding Raster Data
2. Understanding The Concept Of Thematic Data
Discrete data, which is sometimes called thematic, categorical, or discontinuous data, most
often represents objects in both the feature (vector) and raster data storage systems. A discrete
object has known and definable boundaries: it is easy to define precisely where the object
begins and where it ends. A lake is a discrete object within the surrounding landscape. Where
the water’s edge meets the land can be definitively established. Other examples of discrete
objects include buildings, roads, and parcels. Discrete objects are usually nouns.
3. Understanding The Concept Of Continuous Data
A continuous surface represents phenomena in which each location on the surface is a measure
of the concentration level or its relationship from a fixed point in space or from an emitting
source. Continuous data is also referred to as field, nondiscrete, or surface data. One type of
continuous surface is derived from those characteristics that define a surface, in which each
location is measured from a fixed registration point. These include elevation (the fixed point
being sea level) and aspect (the fixed point being direction: north, east, south, and west).
4. Understanding The Concept Of Pictures
In its simplest form, a raster consists of a matrix of cells (or pixels) organized into rows and
columns (or a grid) where each cell contains a value representing information, such as
temperature. Rasters are digital aerial photographs, imagery from satellites, digital pictures, or
even scanned maps.
Data stored in a raster format represents real-world phenomena:
Thematic data (also known as discrete) represents features such as land-use or soils data.
Continuous data represents phenomena such as temperature, elevation, or spectral data such
as satellite images and aerial photographs.
Pictures include scanned maps or drawings and building photographs.
Thematic and continuous rasters may be displayed as data layers along with other geographic
data on your map but are often used as the source data for spatial analysis with the ArcGIS
Spatial Analyst extension. Picture rasters are often used as attributes in tables—they can be
displayed with your geographic data and are used to convey additional information about map
features.
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Be Able To Give An Example As A Raster Used For Base Maps
Be Able to Give An Example As A Raster Used For Surface Maps
Be Able to Give An Example As A Raster Used For Thematic Maps
Be Able to Give An Example As A Raster Used For Attributes For Features
The Purpose Of Storing Data As A Raster
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Give At Least Three Characteristics Of Raster Data
What Are Cells & Why They Are Important In Raster Data
How Are Raster Attribute Tables Formatted
List Three Differences (Vector vs. Raster)
Raster and vector are the two basic data structures for storing and manipulating images and
graphics data on a computer. Major GIS and CAD (Computer Aided Design) software packages
available today are primarily based on one of the two structures, either raster based or vector
based, while they have some extended functions to support other data structures.
Raster images come in the form of individual pixels, and each spatial location or resolution
element has a pixel associated where the pixel value indicates the attribute, such as color,
elevation, or an ID number. Raster images are normally acquired by optical scanner, digital CCD
camera and other raster imaging devices. Its spatial resolution is determined by the resolution
of the acquisition device and the quality of the original data source. Because a raster image has
to have pixels for all spatial locations, it is strictly limited by how big a spatial area it can
represent. When increasing the spatial resolution by 2 times, the total size of a two-dimensional
raster image will increase by 4 times because the number of pixels is doubled in both X and Y
dimensions. The same is true when a larger area is to be covered when using same spatial
resolution.
Vector data comes in the form of points and lines that are geometrically and mathematically
associated. Points are stored using the coordinates, for example, a two-dimensional point is
stored as (x, y). Lines are stored as a series of point pairs, where each pair represents a straight
line segment, for example, (x1, y1) and (x2, y2) indicating a line from (x1, y1) to (x2, y2).
In general, vector data structure produces smaller file size than raster image because a raster
image needs space for all pixels while only point coordinates are stored in vector
representation. This is particularly true in the case when the graphics or images have large
homogenous regions and the boundaries and shapes are the primary interest. Besides the size
issue, vector data is easier than raster data to handle on a computer because it has fewer data
items and it is more flexible to be adjusted for different scale, for example, a projection system
in mapping application. This makes vector data structure the apparent choice for most mapping,
GIS and CAD software packages. Also, topology among graphical objects or items are much
easier to be represented using vector form, since a commonly shared edge can be easily defined
according to its left and right side polygons. On the other hand, this is almost impossible or very
difficult to do with pixels
14. Recognize The Vector & Raster Data Model & Representations
vector data model: [data models] A representation of the world using points, lines, and
polygons. Vector models are useful for storing data that has discrete boundaries, such as
country borders, land parcels, and streets.
raster data model: [data models] A representation of the world as a surface divided into a
regular grid of cells. Raster models are useful for storing data that varies continuously, as in an
aerial photograph, a satellite image, a surface of chemical concentrations, or an elevation
surface.
15. Understanding Raster Spatial Resolution
The spatial resolution of a raster refers to the size of the cells in a raster dataset and the ratio of
screen pixels to image pixels at the current map scale. For example, one screen pixel may be the
result of nine image pixels resampled into one—this is a raster resolution of 1:9. In this case,
every screen pixel has to display nine raster cells, meaning the image is not as clear and
detailed.
A resolution of 1:1, however, means that every screen pixel is displaying exactly one raster cell.
If you zoom in closer than a raster resolution of 1:1, you won’t see any more detail in that
image.
16. Give Examples Of Raster Data Formats
17. Raster & Vector Data Models & Their Advantages
vector data model: [data models] A representation of the world using points, lines, and
polygons. Vector models are useful for storing data that has discrete boundaries, such as
country borders, land parcels, and streets.
raster data model: [data models] A representation of the world as a surface divided into a
regular grid of cells. Raster models are useful for storing data that varies continuously, as in an
aerial photograph, a satellite image, a surface of chemical concentrations, or an elevation
surface.
Raster images are capable of displaying a myriad of colors in a single image and allow for color
editing beyond that of a vector image. They can display finer nuances in light and shading at the
right resolution. Vector images are scalable, so that the same image can be designed once and
resized infinitely for any size application - from business card to billboard.
Raster images cannot be made larger without sacrificing quality. Vector images cannot display
the natural qualities of photographs. Raster images are often large files, while vector images are
relatively lightweight. Raster images are used in web and print, vector images cannot as of this
writing be used in electronic format - they must be converted to a raster first. Vectors display at
the highest resolution allowed by the output device, while rasters blur when blown up.
18. The Difference Between Raster Catalogs, Raster Datasets, Mosaic Datasets
19. Understanding Pixels
20. Understanding Bands
Some rasters have a single band, or layer (a measure of a single characteristic), of data, while
others have multiple bands. Basically, a band is represented by a single matrix of cell values, and
a raster with multiple bands contains multiple spatially coincident matrices of cell values
representing the same spatial area. An example of a single-band raster dataset is a digital
elevation model (DEM). Each cell in a DEM contains only one value representing surface
elevation. You can also have a single-band orthophoto, which is sometimes called a
panchromatic or grayscale image. Most satellite imagery has multiple bands, typically containing
values within a range or band of the electromagnetic spectru
When there are multiple bands, every cell location has more than one value associated with it.
With multiple bands, each band usually represents a segment of the electromagnetic spectrum
collected by a sensor. Bands can represent any portion of the electromagnetic spectrum,
including ranges not visible to the eye, such as the infrared or ultraviolet sections. The term
band originated from the reference to the color band on the electromagnetic spectrum.
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21. What Is RGB
22. What Are A Few Open Data Sources For Raster Data
23. What Is Remote Sensing
Remote sensing is the acquisition of information about an object or phenomenon without
making physical contact with the object and thus in contrast to on site observation. Remote
sensing is a sub-field of geography. In modern usage, the term generally refers to the use of
aerial sensor technologies to detect and classify objects on Earth (both on the surface, and in
the atmosphere and oceans) by means of propagated signals (e.g. electromagnetic radiation). It
may be split into active remote sensing (when a signal is first emitted from aircraft or
satellites)or passive (e.g. sunlight) when information is merely recorded.
24. What Is Electromagnetic Radiation (EM)
Electromagnetic radiation (EM radiation or EMR) is the radiant energy released by certain
electromagnetic processes. Visible light is one type of electromagnetic radiation; other familiar
forms are invisible electromagnetic radiations, such as radio waves, infrared light and X rays.
25. Know The Platforms Of Remote Sensing
26. Identify & Explain The Four Resolutions
There are four major types of resolution in remote sensing:
1. Spatial resolution is dependent on the field of view, altitude, and viewing angle of a sensor.
2. Spectral resolution refers to the number of wavelength regions or bands in the
electromagnetic spectrum to which the sensor is sensitive.
3. Temporal resolution is a measure of how often data are otained for the same area (how
often it is revisited).
4. Radiometric resolution is a measure of the sensitivity of a sensor to differences in the
intensity of the radiation measured.
27. Explain The Concept Behind The Electromagnetic Spectrum
The electromagnetic (EM) spectrum is the range of all types of EM radiation. Radiation is energy
that travels and spreads out as it goes – the visible light that comes from a lamp in your house
and the radio waves that come from a radio station are two types of electromagnetic radiation.
The other types of EM radiation that make up the electromagnetic spectrum are microwaves,
infrared light, ultraviolet light, X-rays and gamma-rays.
28. Define Multispectral
A multispectral image is one that captures image data at specific frequencies across the
electromagnetic spectrum. The wavelengths may be separated by filters or by the use of
instruments that are sensitive to particular wavelengths, including light from frequencies
beyond the visible light range, such as infrared. Spectral imaging can allow extraction of
additional information the human eye fails to capture with its receptors for red, green and blue.
It was originally developed for space-based imaging.[1]
Multispectral images are the main type of images acquired by remote sensing (RS) radiometers.
Dividing the spectrum into many bands, multispectral is the opposite of panchromatic, which
records only the total intensity of radiation falling on each pixel.
29. Define Infrared
having a wavelength just greater than that of the red end of the visible light spectrum but less
than that of microwaves. Infrared radiation has a wavelength from about 800 nm to 1 mm, and
is emitted particularly by heated objects.
30. Define Ultraviolet
having a wavelength shorter than that of the violet end of the visible spectrum but longer than
that of X-rays.
31. What is DEM, DSM, DTM, DHM, and Bathymetry?