5/31/2014

Assignment - 05

Using MATLAB

Submitted To: Sir. Imran Ali Memon

Subject: Geo-Informatics

Submitted By: Atiqa Ijaz Khan

Roll No: Geol-02

MPhil Geomatics

Institute of Geology, university of the Punjab

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ASSIGNMENT - 05

Question.1:

Differentiate the following Matlab commands with example and write shortly about

their application and characteristics.

1. Imshow

It is used to display an image. It tried to fit the image onto the screen. But if is too large

it shows a warning message. It scales an image by using interpolation to re-fit for display.

One can show two images on a same figure plot by ‘imshow’. In comparison to

‘imtool’, ‘imshow’ dose not display automatic pixel values at the bottom.

‘Imshow’ can display images by passing:

Image URL, if not in the current folder,

Complete image name if in the current folder, and,

Variable name in which image is stored previously.

2. Geoshow

It projects and displays the latitude and longitude vectors using projection stored in the

axes. Of not, then it display in the default Plate Carree projection.

To use ‘geoshow’, function ‘shaperead’ must include ‘‘UseGeoCoords’, true’. Otherwise

it will create a mapstruct, with X and Y, without lat and long.

It can go with: points, multipoints, lines, and, polygons

Not a current

folder

Stored in

Workspace

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Displays the lat and long instead

of XY

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3. Mapshow

It display map coordinate xy vectors. The default is to show in lines. If use ‘geoshow’

instead, it will show a warning message in the command window. But displays the

image using ‘mapshow’

XY instead of

lat and long

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Question No.2:

The edge command is used to find edges in intensity image. Use this command on a

image with different parameters and edge techniques. Write down about the differences

and application of all variants of edge command.

Edge detection is an image processing technique for finding the boundaries of objects

within images. It works by detecting discontinuities in brightness. Edge detection is used

for image segmentation and data extraction in areas such as image processing, computer

vision, and machine vision. It let you identify object boundaries in an image.

Common edge detection algorithms include Sobel, Canny, Prewitt, and Roberts

methods.

1st Method (General Syntax):

>> D = imread('D.jpg');

>> G = rgb2gray(D);

>> E = edge(G);

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2nd

Method (Sobel):

>> E = edge(I, 'sobel');

>> imshow(E)

It returns those points where gradient of image is maximum.

By default, ‘edge’ uses the technique ‘sobel’ to detect the edges of an image. It will

display edges for a binary image and gray image. But no for 3D images like, rgb.

3rd Method (Prewitt):

>> E = edge(I, 'prewitt');

>> imshow(E)

4th Method (Roberts):

>> E = edge(I, 'roberts');

>> imshow(E)

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5th Method (Laplacian of ):

>> E = edge(I, 'log');

>> imshow(E)

6th Method (Zero-cross):

>> E = edge(I, 'zerocross');

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>> imshow(E)

7th Method (Canny):

>> E = edge(I, 'canny');

>> imshow(E)

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Sr.

No Algorithms Methods Merits De-merits

01. Sobel Uses 3*3 mask,

separately on x-y axis

Most common 1st

order, simple Sensitive to noise.

02. Prewitt

It’s done by

averaging filter and

1st order

differentiation

Simple 1st order

technique

Less accurate,

sensitive to noise.

03. Roberts

Perform 2*2 mask on

2D spatial gradient

measurement on an

image

Basic 1st order

technique,

Sensitive to noise,

not for 3D images.

04.

LoG

(Laplacian

of

Gaussian)

Use 3*3 mask with

central value to be

+ve, while all other

are –ve, so to have

the sum = 0. Applied

on 2D isotropic

measurements of 2nd

derivative.

Test wide area

around pixels, find

correct places of

edges, highlights

the region of rapid

intensity changes,

and show dark

and white line

along the BLOB.

Not good at curves

and corners, for

good results apply

it to already

smoothen images,

and is independent

of directions.

05. Canny

Use Gaussian filter,

to find out the

maxima threshold

(white), and non-

maxima (black),

incorporating the

directions.

Most Better

powerful,

detection in noise

conditions,

Complex

computations.

Edge Detection Algorithms

Classical-Directional (1st Order)

Sobel Prewitt Roberts

Zero-

crossing

Laplacian

2nd

Directioanl

Derivative

LoG Gaussian

CannyShen_Cast

an

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ASSIGNMENT - 05

Question No.3:

What is world file. Write its extensions and structure of the file, the command to read

and write a world file in Matlab.

A world file is a plain text computer data file used by geographic information

systems to geo-reference raster map images. The file specification was introduced by

ESRI.

The world file adds extension of ‘w’ with the normal used extensions, like tif to tfw.

World files do not specify a coordinate system; this information is generally stored

somewhere else in the raster file itself or in another companion file. The generic

meaning of world file parameters are:

Line 1: A: pixel size in the x-direction in map units/pixel.

Line 2: D: rotation about y-axis.

Line 3: B: rotation about x-axis.

Line 4: E: pixel size in the y-direction in map units, almost always negative.

Line 5: C: x-coordinate of the center of the upper left pixel.

Line 6: F: y-coordinate of the center of the upper left pixel.

A better description of the A, D, B and E parameters would be:

Line 1: A: x component of the pixel width.

Line 2: D: y component of the pixel width.

Line 3: B: x component of the pixel height.

Line 4: E: y component of the pixel height, almost always negative.

All four parameters are expressed in the map units depending on the coordinate system

associated with the raster.

World files describing a map on the Universal Transverse Mercator coordinate

system (UTM) use these conventions:

D and B are usually 0, since the image pixels are usually made to align with the

UTM grid

C is the UTM easting

F is the UTM northing

Units are always meters per pixel

The transformation parameters are stored in the world file in this order:

20.17541308822119 = A

0.00000000000000 = D

0.00000000000000 = B

-20.17541308822119 = E

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424178.11472601280548=C

4313415.90726399607956 = F

If the base name follows the convention of ‘.3’ for extension, then skipped the middle

one to append ‘w’ for world-file extension, like ‘tif’ to ‘tfw’.

If now, then ‘w’ is appended to the full image extension name, like ‘jpeg’ to ‘jpegw’.

If does not contain any extension, then, ‘.wld’ is appended to the base image name.

WORLDFILEWRITE:

The general syntax is:

worldfilewrite (R, worldfilename.extw)

It requires a:

Map raster reference object

Geographic raster reference object

3-by-2 referencing matrix

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WORLDFILEREAD:

The general syntax for ‘worldfileread’ is:

R = worldfileread (worldFileName, coordinatesystemtype, rastersize)

It will construct a spatial reference object. It can be:

Planar for projected coordinates, or,

Geographical for lat-long system.

This can be accessed by ‘coordinateSystemType’ string. The ‘raster-size’ should match

image size corresponding the world-file.

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1. Chao, Yang. "A comparison of medical image analysis algorithms for edge

detection." 2010.

2. H.S. Bhadauria, Annapurna Singh, Anuj Kumar. "Comparison between Various

Edge Detection Methods on Satellite Images." Comparison between Various

Edge Detection Methods on 3, no. 6 (2013): 324-328.

3. Hao Yuan Kueh, Eugenio Marco, and Sivaraj Sivaramakrishnan. "Image analysis

for biology." n.d.

4. MapInfo Products Knowledge Base. n.d.

http://testdrive.mapinfo.com/TechSupp/MIProd.nsf/05edbd1e930f8d4d85257

12500748199/61409cb5b20c23f28525771a0054558a?OpenDocument

(accessed May 31, 2014).

5. MathWorks. "mathworks." Mapping Toolbox User Guide. n.d. (accessed May

2014).

6. Morris, Dr. Tim. Image Processing with MATLAB. n.d. (accessed May 2014).

7. Pushpajit A. Khaire & Dr. N. V. Thakur. "A Fuzzy Set Approach for Edge

Detection." International Journal of Image Processing 6, no. 6 (2012): 403-412.

8. Raman Maini & Dr. Himanshu Aggarwal. "Study and Comparison of Various

Image Edge Detection Techniques." International Journal of Image Processing 3,

no. 1 (n.d.).

9. Tauler, Romà. Mapping. n.d. (accessed May 2014).

10. Using ArcIMS 9.2 Understanding World File. n.d.

http://webhelp.esri.com/arcims/9.2/general/topics/author_world_files.htm

(accessed May 31, 2014).