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Data Models, Pixels, and Satellite Bands
Introduction to Digital Data and Imagery
Understand the differences between raster and vector data.
What are digital numbers (DNs) and what do they represent in remotely sensed data?
What are satellite bands?Be able to define spatial and spectral
resolution.What are some advantages and disadvantages
of high spatial resolution?What are some advantages and disadvantages
of high spectral resolution?
Learning Objectives
Data Models: Raster vs. Vector DataRaster Data
Images are grids of cells or “pixels”
Each pixel is represented by number called the digital number (DN)
A pixel represents an area on the ground
Vector DataPoints, lines, and
areasDefined by x,y
coordinates and info about “connectedness”
Attributes can describe the features they represent
100
10 250
0 122
35 75 255
5 90 92
112
12 11 2 3 200
78 200
20 75 93
187
202
35 87 90 90 140
144
52 68 89
2 3 23 76 56 62 178
142
24 53 1
0 0 10 35 57 63 242
137
36 25 0
Raster Grid w/ DNs
Point
Line or Arc
Area or Polygon
Vector Features
Data ModelImaginary matrix (row & column format) is
placed on the feature (e.g., the ground)Some phenomenon (e.g. amount of light) is
measuredA value (digital number) representing the
amount of light is assigned to each grid cell (pixel).
Somewhere on earth
Raster grid is placed
Raster data
32 47 67 93 11
105 79 35
23
11
56
43
89
21
213
245
201
179
136 155
55203
163 63
211 189
145
109122202
Real world >>> Raster value
For remote sensing, the total amount of EMR from the area of a pixel on the ground is recorded as a digital number (DN).
Depending on the intensity of the EMR, a numeric value is assigned to each pixelLow or None - Lowest value (dark)High - Maximum value (bright)Others - Scaled in between (gray)
Digital Image DataDigital images are matrices of digital
numbers (DNs).Satellite bands capture light from different
wavelength regions.There is one layer (or matrix) for each
satellite band.Each band covers the same area on the
groundEach DN corresponds to one pixel in one
bandIf there are 6 bands each pixel will be
described by 6 DNs, one for each bandThe DNs control how a computer displays an
image on your computer screen
Images are presented as 2-d grids. Each pixel (one square) has a location (x,y).
Position of pixel (x,y) often describe in terms of rows and columns but can be translated (projected) into other coordinate systems (e.g., latitude/longitude, UTM, etc.)
0 0 0 2
0 2 3 4
2 2 6 9
2 2 9 12
F(2,3)
F(4,1)
What are digital numbers (DNs)?DNs are relative measures of radianceDNs are NOT reflectanceDNs can be converted to ground reflectance
if you know atmospheric properties, etc.The range of DNs depends on the radiometric
resolution of the recording instrumentA common range of DNs is 0 – 255.
Resolution
Types of ResolutionSpatialSpectralRadiometricTemporal
Spatial ResolutionThe dimension of one side of a single pixelThe extent of the smallest object on the
ground that can be distinguished in the imagery
Determined by the Instantaneous Field of View of satellite instruments (IFOV)
Determined by altitude, camera (lens), and film characteristics for air photos.
Spatial Resolution
Raster grid sizefiner
Coarser
Available Resolution
Satellites: ~ .61 m to > 1 kmAir photos ~ <0.6 m to large.
Satellite data resolutionMODIS: 250 - 1000 mLandsat MSS: 80 mLandsat TM5, 7, 8: 30 mSPOT: 20 m ASTER: 15 mIRS Pan: 5 mWorldview 3 Pan: 31 cm (!!)
Quickbird (Digital Globe, Inc.)
~ 2.4 m spatial resolution in multispectral bands.
MODIS
500 m spatial resolution
So…what about in the movies?
Zoom and Enhance! (YouTube)
The Truth!
Group discussion: Can you extract (zoom and enhance!) information from an image when the information is in an area smaller than a pixel?
Spectral ResolutionHow finely an instrument “divides up” the
range of wavelengths in the electromagnetic spectrum
How many spectral “bands” an instrument records Landsat 8
bands (in red) overlaid on plant spectral reflectance.
https://landsat.usgs.gov/tools_spectralViewer.php
Case 1Measure the EMR across a wide rangeE.g., the visible portion of EMRAssign a single DN for sum of all visible light
energy hitting the sensorAnalogous to black and white (panchromatic)
filmCalled a panchromatic band
blue
green
red
0.4 0.70.60.5UV Near-infrared
Case 1
Case 2
Measure EMR across narrower ranges
E.g., Blue, green and red bands
Assign a DN for each of these wavelength ranges to create 3 bands
Case 2
blue
green
red
0.4 0.70.60.5UV Near-infrared
Coarser (lower) Spectral Resolution
Finer (higher) Spectral Resolution
RGB
Red Green Blue
400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 25000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500
0.0
0.2
0.4
0.6
0.8
High Spectral Resolution
Low Spectral Resolution
Wavelength (nm)
Wavelength (nm)
Refl
ect
an
ceR
efl
ect
an
ce
Spectral Resolution
Remotely sensed imagery are RASTER data composed of grids of pixels organized in bands (layers)
Size of pixels is called spatial resolution of sensor
Number of bands is called spectral resolution of sensor
Digital numbers are associated with pixels and tell you relatively how much light came from that area on the ground to the satellite sensor.
Summary