Fundamentals of Remote Sensing- A training module

Fundamentals Remote Sensing

The science and art of acquiring of information about an object without being in physical contact with it

Dr. Nishant Sinha

Remote Sensing

As you view the screen of your computer monitor, you are actively engaged in remote

sensing.

HOW ?

THE ANSWER IS

A physical quantity (light) emanates from the screen,

which is a source of radiation. The radiated light passes over a distance, and thus is "remote" to

some extent.

Remote Sensing

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2

3 5

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1. energy source

2. atmospheric interaction

3. ground object

4. data recording / transmission

5. ground receiving station

6. data processing

7. expert interpretation / data users

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Balloon Remote Sensing, Paris, 1858

Pigeon Remote Sensing

Actual Pigeon Pictures

Wilbur Wright and his first aerial photograph of France.

Apollo Spacecraft Mission

Satellite Remote Sensing

LandSAT Satellite

Ox-Bow of the Mississippi

Indian Remote Sensing (IRS) Satellite

Bihar State Map on AWiFS Data

Remote Sensing Sensors

True color film

Infrared film

Landsat Images

Quickbird Image

▪ Area is covered by grid with (usually) equal-sized cells

▪ Cells often called pixels (picture elements); raster data often called image data

▪ Attributes are recorded by assigning each cell a single value based on the majority feature (attribute) in the cell, such as land use type.

▪ Easy to do overlays/analyses, just by ‘combining’ corresponding cell values: “yield= rainfall + fertilizer” (why raster is faster, at least for some things)

▪ Simple data structure:– directly store each layer as a single table

(basically, each is analagous to a “spreadsheet”)– computer data base management system not required (although many raster GIS systems incorporate them)

Representing Data using Raster Model

corn

wheat

fruit

clov

er

fruitoats

Raster Array Representations

▪ Raster data comprises rows and columns, by one or more characteristics or arrays – elevation, rainfall, & temperature; or multiple spectral channels (bands) for remote sensed

data– how organise into a one dimensional data stream for computer storage & processing?

▪ Band Sequential (BSQ)– each characteristic in a separate file

– elevation file, temperature file, etc.

– good for compression

– good if focus on one characteristic

– bad if focus on one area

▪ Band Interleaved by Pixel (BIP)– all measurements for a pixel grouped together

– good if focus on multiple characteristics of geographical area

– bad if want to remove or add a layer

▪ Band Interleaved by Line (BIL)– rows follow each other for each characteristic

File 1: Veg A,B,B,BFile 2: Soil I,II,III,IVFile 3: El. 120,140,150,160

A,I,120, B,II,140 B,III,150 B,IV,160

A,B,I,II,120,140 B,B,III,IV,150,160

Note that we start in lower left. Upper left is alternative.

A B

B B

III IV

I II 150 160

120 140Elevation

Soil

Veg

The generic raster data model is actually implemented in several different computer file formats:

▪ GRID is ESRI’s proprietary format for storing and processing raster data

▪ Standard industry formats for image data such as JPEG, TIFF and MrSid formats can be used to display raster data, but not for analysis (must convert to GRID)

▪ Georeferencing information required to display images with mapped vector data– Requires an accompanying “world” file which provides locational

information

File Formats for Raster Spatial Data

Image Image File World FileTIFF image.tif image.tfwBitmap image.bmp image.bpwBIL image.bil image.blwJPEG image.jpg image.jpw

Remote Sensing: Imagery Types

TES1 m

Quickbird

61cm

IRS 1D23.5 m

IRS 1D5.8 m

High Resolution Imagery

Low Resolution Imagery

Panchromatic Imagery

Multi-spectral Imagery

Resolution

▪ The Ability to Discriminate

▪ Types of Resolution

– Spatial: Discrimination by Distance

– Spectral: Discrimination by Wave length

– Radiometric: Discrimination by energy levels

– Temporal: Discrimination by Time

5.8m5.8m

Radiometric Resolution8-bit (0-255)

Spectral Resolution0.4-0.7 μm

Day 1Day 48

Day 96

What is an image?

▪ Data that are organized in a grid of columns and rows

▪ Usually represents a geographical area

X-axis

Y-axis

An image refers to any pictorial representation, regardless of what wavelengths or remote sensing device has been used to detect and record the electromagnetic energy.

A photograph refers specifically to images that have been detected as well as recorded on photographic film.

Based on these definitions, we can say that all photographs are images, but not all images are photographs.

Difference between Image and Photographs

Pixels

▪ Resulting images are made of a grid of pixels

• Each pixel stores a digital number (DN) measured by the sensor

• Represents individual areas scanned by the sensor

• The smaller the pixel, the easier it is to see detail

Continuous data

Two types:

• Panchromatic ( 1 Band/layer)

• Multispectral ( 2 or more Bands)

Viewing continuous images

▪ Each band or layer is viewable as a separate image

Thematic Mapper Band 1Band 4

Band 5

Blue

Green

Red

NIR

SWIR

Part of spectru

m

Monitorcolor guns

Viewing images

▪ Three bands are viewable simultaneously

Band4

Band3

Band2

Band4

Band5

Band3

Band1

Band2

Band3

Geomteric Corrections

▪ All remote sensing imagery inherently subject to geometric distortions caused by various factors

▪ Geometric corrections intended to compensate for these distortions

▪ Required so that geometric representation of the imagery is as close as possible to the real world

▪ Geometric registration of the imagery to a known ground coordinate system must be performed

▪ Radiometric Corrections– changing the image data BVs to correct for errors or

distortions▪ atmospheric effects (scattering and absorption)▪ sensor errors

▪ Geometric Corrections– changing the geometric/spatial properties of the

image data – Also called image rectification or rubber sheeting

Image Preprocessing

Geometric Registration

▪ Image-to-map registration– Involves identifying the image coordinates (i.e. row, column) of

several clearly discernible points, called ground control points (or GCPs), in the distorted image (A - A1 to A4), and matching them to their true positions in ground coordinates (e.g. latitude, longitude).

– True ground coordinates are measured from a map (B - B1 to B4), either in paper or digital format

▪ Image-to-image registration – Performed by registering one (or more) images to another image,

instead of geographic coordinates

▪ Several types of transformations applied on image co-ordinates to transform into real world coordinates:– Plane transformations - keep lines straight, being on the first order– Curvilinear (polynomial) - higher order transformations that do not

necessarily keep lines straight and parallel– Triangulation.– Piecewise transformations - Break the map into regions, apply

different transformations in each region

Geometric Corrections

▪ All remote sensing imagery inherently subject to geometric distortions caused by various factors

▪ Geometric corrections intended to compensate for these distortions

▪ Required so that geometric representation of the imagery is as close as possible to the real world

▪ Geometric registration of the imagery to a known ground coordinate system must be performed

▪ Distortions factors– the perspective of the

sensor optics– the motion of the scanning

system– the motion of the platform– the platform altitude– attitude, and velocity– the terrain relief and– the curvature and rotation

of the Earth

▪ Distortions type– Systematic (predictable in

nature) ▪ Accounted through

accurate modeling of sensor and platform motion and

▪ Geometric relationship of the platform with the Earth

– Unsystematic (random) errors cannot be modeled and corrected

Earth Rotation Altitude Variation Pitch Variation

Spacecraft Velocity Roll Variation Yaw Variation

Non Systemat

ic Distortio

ns

Systematic

Distortions

Image distortions

Scanner distortions

Actual Velocity

Nominal Velocity

Mirror Angle

time

Mirror velocity variations

Scan Skew

Geometric Correction

▪ Four Basic Steps of Rectification1. Collect ground control points (GCPs)2. “Tie” points on the image to GCPs.3. Transform all image pixel coordinates using

mathematical functions that allow “tied” points to stay correctly mapped to GCPs.

4. Resample the pixel values (BVs) from the input image to put values in the newly georeferenced image

Geometric Correction

▪ Three Types of Resampling– Nearest Neighbor - assign

the new BV from the closest input pixel. This method does not change any values

– Bilinear Interpolation - distance-weighted average of the BVs from the 4 closest input pixels

– Cubic Convolution - fits a polynomial equation to interpolate a “surface” based on the nearest 16 input pixels; new BV taken from surface

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2

3

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2

3

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Image Enhancements

▪ Procedures of making a raw image more interpretable for a particular application

▪ Improve the visual impact of the raw remotely sensed data on the human eye

▪ Classification– Contrast (global) enhancement: Transforms raw data using statistics

computed over whole data set▪ Examples - Linear contrast, histogram equalized and piece-wise contrast

stretch

– Spatial (local) enhancement - Local conditions considered only that vary over image▪ Examples - Image smoothing and sharpening

Image Enhancements▪ Procedures of making a raw image

more interpretable for a particular application


▪ Contrast (global) enhancement: Transforms raw data using statistics computed over whole data set (Examples - Linear contrast, histogram equalized and piece-wise contrast stretch)

▪ Spatial (local) enhancement - Local conditions considered only that vary over image (Examples - Image smoothing and sharpening)

Image Enhancement: Example

▪ Contrast Enhancement - “stretching” all or part of input BVs from the image data to the full 0-255 screen output range

Image Fusion

LISS III PAN

Brovey Multiplicative

PCA

Wavelet

Image Classification

▪ To label the pixels in the image with meaningful information of the real world.

▪ Classification of complex structures from high resolution imagery causes obstacles due to their spectral and spatial heterogeneity

▪ Two types– Unsupervised classification– Supervised classification

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Supervised vs. Unsupervised Approaches

– Unsupervised: statistical "clustering" algorithms

used to select spectral classes inherent to the data,

more computer-automated

Posterior Decision

– Supervised: image analyst "supervises" the selection

of spectral classes that represent patterns or land

cover features that the analyst can recognize

Prior Decision

Edit/evaluate signatures

Select Training fields

Classify image

Evaluate classification

Identify classes

Run clustering algorithm

Evaluate classification

Edit/evaluate signatures

Supervised Unsupervisedvs

Image Enhancements

▪ Procedures of making a raw image more interpretable for a particular application


▪ Classification– Contrast (global) enhancement: Transforms raw data using statistics

computed over whole data set▪ Examples - Linear contrast, histogram equalized and piece-wise contrast

stretch

– Spatial (local) enhancement - Local conditions considered only that vary over image▪ Examples - Image smoothing and sharpening

Band Combinations

▪ Features can become more obvious

Vegetation

Urban

2,3,1 (RGB)4,3,2 (RGB)4,5,3 (RGB)

Keys to Image Interpretation

▪ Shape

▪ Size

▪ Shadows

▪ Tone

▪ Color

▪ Texture

▪ Pattern

Interpretation Principles

•Shape

•Size

•Shadow

•Tone/Color

•Texture

•Pattern

•Relationship to Surrounding Objects

▪ The photo on the right is a black and white photo of the City of Ithaca and the Cornell University campus taken in 1991. More specifically, it was taken on April 4, 1991 (look in the upper left hand corner).

▪ So lets take a quick tour of the photograph

Image Interpretation

▪ Size: the size of an object is one of the most useful clues to its identity. Also, understanding the size of one object may help us understand the sizes of other objects.

▪ For example, most of us have a feeling for the size of a baseball field, and football field. When we observe these objects on a photograph, it will help us to understand the sizes of other objects on the photograph.

▪ For example, on another part of the photograph we have a trailer park. This could easily be confused with a parking lot, but when we understand the size of the objects we will realize that the objects in the trailer park are much too large to be cars.


▪ Shape: Shapes can often give away an object’s identity. For example, a cloverleaf is a very distinctive feature of a highway, while a stream’s meandering gives away its identity.

▪ And again, the baseball diamond we just looked at also has a distinctive shape.


▪ Shadow: shadows often give us an indication of the size and shape of an object. When we look at aerial photographs we often see a vantage point we are not used to: an overhead view.

▪ Shadows can let us “cheat” alittle to see the side of an object. The photos on the right show the Cornell Theory Center, which casts a rather large shadow, indicating the building size, and a water tower on one of the farms on campus. If you look closely, you can see the “legs” of the watertower.


▪ Shadow: while shadows are helpful, they can also be a hindrance. As we try to look down into the gorge on the Cornell campus, we can see very little due to the shadows cast.


▪ Tone: You can see the tonal contrast between Cayuga Lake and the land area. Also, there is good tone representation for wet or dry soils.


▪ Texture: In this photo we see the Cornell Plantations and Botanical Garden, as well as the experimental agricultural plots. Especially in the Plantations, you will see the different textural characteristics between the mowed lawns and the grassy areas. Notice too, the small pond in the Plantations (an example of tone)

▪ Additionally, around another natural area on campus you can see the textural difference of trees vs. more of a grassland area.

▪ And again, as you look at the agricultural plots you will notice a different texture from the forested areas.

▪ Finally, in the golf course shown below there are obvious patterns between managed lawns vs. the unmanaged lawns, in addition to the tonal differences between the lawns and sand traps.


▪ Pattern: There are so many examples related to pattern. These would include the rectilinear pattern of the older, urban neighborhoods in Ithaca, the straight lines of trees in an orchard, the rectilinear shape of the experimental agricultural plots, and the configuration of a parking lot.

▪ Also, the pattern of the golf course with greens, tees, traps, and fairways is very easy to spot.


▪ Pattern: the drainage pattern for a particular property on this photo is easy to see. Also, because the drainage is relatively straight, we can assume that a moderate to steep slope exists, as water did not have much opportunity to meander.


▪ Relationship: observing relationships on photographs is one of the most fun observations. For example, a school and a plaza are interpreted differently due to relationships:– While both have many large

structures on them, schools typically have playing fields

– Also, plazas usually have larger parking areas

▪ Here we see the East Hill Shopping Plaza (no athletic fields, but a campus of buildings), and the Ithaca High School campus (with athletic fields)


▪ Relationship: here is another example of relationship that shows a middle school and an elementary school. Notice that it have buildings like the high school, and a parking lot, but no real athletic fields to speak of. What it does have, however, is what appears to be a playground, and is surrounded by a residential community.

▪ The structures on the top are an apartment complex. They could be tractor trailers, but “size” gives them away. They are too large to be tractor trailers when you consider the size of the schools below.

▪ Notice that just north of the apartment complex is a large pool. How do we know it’s a pool, well, the tone gives us a clue…


Apartments

School

School

▪ interpretation is putting together our observations bit by bit to form a coherent understanding of the image. For instance, identifying the water treatment plant forces us to use shape, pattern, tone, and relationship to make the connection:– We see the water holding

areas in black (tone)– We see the large tanks

(shape)– And when you’ve seen one

treatment plant, you’ve seen them all (pattern)!!

▪ Notice that across the water is a park. Why do we know it’s a park? Well, again, we see multiple ball fields, not enough buildings to be a school, and a very large pool.


Acknowledgement

These slides are aggregations for better understanding of GIS. I acknowledge the contribution of all the authors and photographers from where I tried to accumulate the info and used for better presentation.

Author’s Coordinates:Dr. Nishant SinhaPitney Bowes Software, [email protected]

Fundamentals of Remote Sensing- A training module

Technology

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