Gis unit 3

OCE 552 - Geographic
Information System

UNIT III DATA INPUT AND
TOPOLOGY 9
Scanner - Raster Data Input – Raster
Data File Formats – Vector Data Input –
Digitiser – Topology - Adjacency,
connectivity and containment –
Topological Consistency rules – Attribute
Data linking – ODBC – GPS - Concept
GPS based mapping.

Scanners
 Scanning converts paper maps into digital
format by capturing features as individual
cells, or pixels, producing an automated
image.
 A scanner can be thought of as an electronic
input device that converts analog information
of a document like a map, photograph or an
overlay into a digital format that can be used
by the computer.
 Scanning automatically captures map
features, text, and symbols as individual cells,
or pixels, and produces an automated image.

Working of a scanner
 Scanner head
◦ Move along the length of the scanner
◦ The scanner head contains either a charged-couple
device (CCD) sensor or a contact image (CIS)
sensor.
◦ A CCD consists of a number of photosensitive cells
or pixels packed together on a chip.
◦ The most advanced large format scanners use
CCD’s with 8000 pixels per chip for providing a very
good image quality.

 While scanning a bright white light from the scanner
strikes the image to be scanned and is reflected onto the
photosensitive surface of the sensor placed on the
scanner head.
 Each pixel transfers a graytone value (values given to
the different shades of black in the image ranging from 0
(black) – 255 (white) i.e. 256 values to the scanboard
(software).
 The software interprets the value in terms of 0 (Black) or
1 (white), thereby, forming a monochrome image of the
scanned portion.
 As the head moves ahead, it scans the image in tiny
strips and the sensor continues to store the information
in a sequential fashion.
 The software running the scanner pierces together the
information from the sensor into a digital form of the

 Scanning a colour image is slightly different in which the
scanner head has to scan the same image for three
different colours i.e. red, green, blue.
 In older colour scanners, this was accomplished by
scanning the same area three times over for the three
different colours. This type of scanner is known as three-
pass scanner.
 However, most of the colour scanners now scan in one
pass scanning all the three colours in one go by using
colour filters.
 In principle, a colour CCD works in the same way as a
monochrome CCD. But in this each colour is constructed
by mixing red, green and blue. Thus, a 24-bit RGB CCD
presents each pixel by 24 bits of information.
 Usually, a scanner using these three colours (in full 24
RGB mode) can create up to 16.8 million colours.

Types of scanners
 Hand-held scanner
◦ Hand-held scanners although portable, can only
scan images up to about four inches wide.
◦ They require a very steady hand for moving the
scan head over the document.
◦ They are useful for scanning small logos or
signatures and are virtually of no use for scanning
maps and photographs.

Types of scanners
 Flat-bed scanner
◦ The most commonly used scanner is a flatbed
scanner also known as desktop scanner.
◦ It has a glass plate on which the picture or the
document is placed. The scanner head placed
beneath the glass plate moves across the picture
and the result is a good quality scanned image.
◦ For scanning large maps or toposheets wide format
flatbed scanners can be used.

Types of scanners
Drum scanner
 The drum scanners are mostly used by the
printing professionals.
 In this type of scanner, the image or the document
is placed on a glass cylinder that rotates at very
high speeds around a centrally located sensor
containing photo-multiplier tube instead of a CCD
to scan.
 Prior to the advances in the field of sheet fed
scanners, the drum scanners were extensively
used for scanning maps and other documents.

Types of scanners
Sheet-fed scanner
 Sheet fed scanners work on a principle similar to
that of a fax machine.
 In this, the document to be scanned is moved
past the scanning head and the digital form of the
image is obtained.
 Disadvantage: it can only scan loose sheets and
the scanned image can easily become distorted if
the document is not handled properly while
scanning. However, the new generation of the
wide format sheet fed scanners has overcome
this problem and have become indispensable for
scanning maps, imageries and other large sized
documents.

Features of scanners
1. The speed of a scanner is dependent on the size
of the document scanned. Larger the document,
longer the time it takes to scan. Similarly,
resolution also affects the scanning speed.
Scanning in high resolution is slow and requires
much greater time than scanning in low resolution.
2. Resolution is a very important property of a
scanner as well as a scanned image. It is the
degree of sharpness of a displayed character or
image. For scanners, resolution is generally
expressed in dots per linear inch. Thus, 300 dpi
means 90,000 dots per square inch. Generally,
increasing the scan resolution means increasing
the size of the image. Large images, in turn,
means larger memory consumption. Hence a
trade-off must be maintained between image

Features of scanners
3. A scanner interface is a software that lets the
scanner hardware communicate with the
application that initiates a scan process.
Scanners are generally available in two
interfaces:
◦ Small Computer System Interface (SCSI)
scanners help in fast scans. The technically
advanced scanners usually have this
interface.
◦ Parallel Interface is used by some low-end
scanners. These are slower than the SCSI
interface scanners.

Types of scanning
Scanning captures map features, text, and
symbols as individual cells, or pixels, and produce
an automated image. Based on the document to
be scanned there are different scanning
procedures followed.
1. Black and White Raster Scanning:
◦ Image scanned in B&W Black and white or
“binary” scanning is the simplest method of
converting any document and can be
performed on line drawings, reduced media,
text or any one colour document.
 Applications
◦ Archival Drawing Libraries
◦ Electronic Document Distribution

Types of scanning
2. Grey Scale and Colour Raster Scanning:
◦ Image scanned in color Gray scale and
(especially) colour images can be quite large. It
must be made sure that the system is capable of
handling files whose size is often measured in
tens of megabytes. Because virtually every pixel is
populated with a value, an attempt to compress
the file results in little or no reduction in file size.
 Applications
◦ Aerial photography
◦ Toposheets
◦ Navigation charts (air and nautical)
◦ Full colour maps
◦ Brochures and artwork
◦ Cartographic base data for “high-end” mapping
systems

Types of scanning
 Rather than using black and white printing
plate separates, separate images of map
features are created which can be
distinguished by colour. For example,
elevation contours can be extracted from a
colour image of a toposheet. This process is
much faster, and thus more cost effective,
than attempting to capture data directly from
the colour image.
 Applications
◦ Contour maps
◦ Road maps
◦ Hydrological maps
◦ Environmental maps
◦ Oil and gas mapping

Processing of Scanned
Document
 Scanning results in conversion of the
image into an array of pixels thereby
producing an image in raster format. A
raster file is an image created by a series
of dots (called “pixels”) that are arranged
in rows and columns. A scanner captures
the image by assigning a row, a column,
and a colour value (black or white, a grey
scale, or a colour) to each dot.

Raster Data Input /
Raster Data File
formats

Raster File
Formats
 Widely used JPEG, TIFF, and PNG formats
 JPEG and TIFF – Used in digital cameras
- 8 bit data
 JPEG (Joint Photographic Experts Group) – Lossy
compression
 File extension is jfw
 TIFF(Tagged Image File Format) – Either lossy or
lossless
 File extension is tiff
 PNG (Portable Network Graphics )- Either lossy or
lossless
 Viewed better in web-based browsers such as
Internet Explorer, Mozilla Firefox, Netscape, and
Safari.
 File extension is png

 MrSID (Multiresolution Seamless
Image Database) format is lossless
compression format was developed by
LizardTech, Inc., for use with large
aerial photographs or satellite images.
 The MrSID format is frequently used
for visualizing orthophotos.

 The proprietary ECW (Enhanced
Compression Wavelet) format also includes
georeferencing information within the file
structure.
 This lossy compression format was
developed by Earth Resource Mapping and
supports up to 255 layers of image
information.
 Due to the potentially huge file sizes
associated with an image that supports so
many layers, ECW files represent an
excellent option for performing rapid
analysis on large images while using a
relatively small amount of the computer’s
RAM (Random Access Memory), thus
accelerating computation speed.

 Like the TIN vector format, some raster file
formats include the USGS DEM, USGS SDTS,
and DTED file formats. The USGS DEM (US
Geological Survey Digital Elevation Model) is a
popular file format due to widespread
availability, the simplicity of the model, and the
extensive software support for the format.
Digital Surface Model (left) and Digital
Terrain Model (right)

Vector Data Input
 The most common vector file format is the
shapefile.
 Shapefiles, developed by ESRI in the early 1990s
for use with the dBASE III database management
software package in ArcView 2, are simple,
nontopological files developed to store the
geometric location and attribute information of
geographic features.
 Shapefiles are incapable of storing null values, as
well as annotations or network features.
 Field names within the attribute table are limited to
ten characters, and each shapefile can represent
only point, line, or polygon feature sets.
 Supported data types are limited to floating point,
integer, date, and text.
 Shapefiles are supported by almost all commercial
and open-source GIS software.

File
Extension
Purpose
SHP* Feature geometry
SHX* Index format for the feature geometry
DBF* Feature attribute information in dBASE IV format
PRJ Projection information
SBN and SBX Spatial index of the features
FBN and FBX Read-only spatial index of the features
AIN and AIH Attribute information for active fields in the table
IXS Geocoding index for read-write shapefiles
MXS Geocoding index for read-write shapefiles with
ODB format
ATX Attribute index used in ArcGIS 8 and later
SHP.XML Metadata in XML format
CPG Code page specifications for identifying character

Digitizers
 Digitizing in GIS is the process of converting
geographic data either from a hardcopy or a scanned
image into vector data by tracing the features. During
the digitizing process, features from the traced map
or image are captured as coordinates in either point,
line, or polygon format.

Types of Digitizing
 Manual digitizing involves tracing geographic
features from an external digitizing tablet
 Heads up digitizing (also referred to as on-screen
digitizing) is the method of tracing geographic
features from another dataset (usually an aerial,
satellite image, or scanned image of a map)
directly on the computer screen
 Automated digitizing involves using image
processing software that contains pattern
recognition technology to generated vectors.
3/9/2022 27

Procedure for digitizing a paper
map using a manual digitizer
 Registration
 Digitizing point features
 Digitizing line features
 Digitizing area (polygon) features
 Adding attribute information
3/9/2022 28

Registration
 The map to be digitized is fixed firmly to the table top with sticky tape.
Five or more control points are identified (usually the four corners of
the map sheet and one or more grid intersections in the middle).
 The geographic co-ordinates of the control points are noted and their
locations digitized by positioning the cross-hairs on the cursor exactly
over them and pressing the ‘digitize’ button on the cursor.
 This sends the co-ordinates of a point on the table to the computer
and stores them in a file as ‘digitizer co-ordinates’. These co-
ordinates are the positions of the cursor cross-hairs relative to the
wire mesh in the table.
 They are usually stored as decimal inches or centimetres from the
bottom left-hand corner of the table.
 Later, the geographic co-ordinates of the control points are used to
transform all digitizer co-ordinates into geographic co-ordinates.
Therefore, it is essential that the map is carefully registered on the
digitizer table to ensure an accurate transformation of digitized
features from digitizer to geographic co-ordinates. Once the map has
3/9/2022 29

Digitizing point features
 Point features, for example spot
heights, hotel locations or
meteorological stations, are recorded
as a single digitized point. A unique
code number or identifier is added so
that attribute information may be
attached later. For instance, the hotel
with ID number ‘1’ would later be
identified as ‘Mountain View’.
3/9/2022 30

Digitizing line features
 Line features (such as roads or rivers) are
digitized as a series of points that the software
will join with straight line segments. In some GIS
packages lines are referred to as arcs, and their
start and end points as nodes. This gives rise to
the term arc–node topology, used to describe a
method of structuring line features. As with point
features, a unique code number or identifier is
added to each line during the digitizing process
and attribute data attached using this code. For
a road, data describing road category, number of
carriageways, surface type, date of construction
and last resurfacing might be added.
3/9/2022 31

Digitizing area (polygon)
features
 Area features or polygons, for example
forested areas or administrative
boundaries, are digitized as a series of
points linked together by line segments in
the same way as line features. Here it is
important that the start and end points join
to form a complete area. Polygons can be
digitized as a series of individual lines,
which are later joined to form areas. In
this case it is important that each line
segment is digitized only once.
3/9/2022 32

Adding attribute information
 Attribute data may be added to digitized
polygon features by linking them to a
centroid (or seed point) in each polygon.
These are either digitized manually (after
digitizing the polygon boundaries) or created
automatically once the polygons have been
encoded. Using a unique identifier or code
number, attribute data can then be linked to
the polygon centroids of appropriate
polygons. In this way, the forest stand may
have data relating to tree species, tree ages,
tree numbers and timber volume attached to
a point within the polygon.
3/9/2022 33

3/9/2022 35
Point mode
 In point mode the user begins digitizing
each line segment with a start node, records
each change in direction of the line with a
digitized point and finishes the segment with
an end node. Thus, a straight line can be
digitized with just two points, the start and
end nodes. For more complex lines, a
greater number of points are required
between the start and end nodes. Smooth
curves are problematic since they require an
infinite number of points to record their true
shape. However, some digitizing packages
allow the user to record smooth curves as
mathematically defined splines or Bézier

3/9/2022 36
Stream mode
 In stream mode the digitizer is set up to
record points according to a stated time interval
or on a distance basis. Once the user has
recorded the start of a line the digitizer might be
set to record a point automatically every 0.5
seconds and the user must move the cursor
along the line to record its shape. An end node
is required to stop the digitizer recording further
points. The speed at which the cursor is moved
along the line determines the number of points
recorded. Thus, where the line is more complex
and the cursor needs to be moved more slowly
and with more care, a greater number of points
will be recorded. Conversely, where the line is
straight, the cursor can be moved more quickly
and fewer points are recorded.

3/9/2022 37
Advantages of Digitizing
 Low initial capital cost
 Flexible and adapts to different types
of data
 Easily mastered skill
 Digitizing devices are reliable
 Generally the quality of data is high

3/9/2022 38
Problems with Digitizing
 Paper maps are unstable, they stretch or shrink,
sometimes while they are on the digitizing table.
 The accuracy depends on the dedication of the
operator and his training and skill.
 Accuracy also depends on the quality of the
source documents.
 Paper maps weren’t prepared “digitally” correct,
but to visually impart information, for example if
railroads, highways and tunnels pass through a
mountain pass the pass may be drawn larger to
accommodate the drawing.

3/9/2022 39
Scanning Technology
 Converts paper maps into digital
format by capturing features as cells,
or pixels.
 Cells are captured using a scanner
head made up of photosensitive cells.
 Advanced large format scanners have
heads with 8000 photosensitive cells
 Each sensor is able to record a pixel
rated between 0 (black) to 255 (white)
and any graytones between.

Topology
A topology is a mathematical procedure
that describes how features are spatially
related and ensures data quality of the
spatial relationships. Topological
relationships include following three basic
elements:
 I. Connectivity: Information about
linkages among spatial objects
 II. Contiguity: Information about
neighboring spatial object
 III. Containment: Information about
inclusion of one spatial object within
another spatial object

Connectivity
Arc node topology defines
connectivity –
1. Arcs are connected to each other if
they share a common node. This is
the basis for many network tracing
and path finding operations.
2. Arcs represent linear features and the
borders of area features.
3. Every arc has a from-node which is
the first vertex in the arc and a to-node
which is the last vertex.

 Nodes can, however, be used to
represent point features which
connect segments of a linear feature
(e.g., intersections connecting street
segments, valves connecting pipe
segments).
Node showing intersection

Arc-Node Topology with list
 Arc-node topology is supported through an
arc-node list. For each arc in the list there
is a from node and a to node. Connected
arcs are determined by common node
numbers.

Contiguity
 Polygon topology defines contiguity. The
polygons are said to be contiguous if they
share a common arc.
 Contiguity allows the vector data model to
determine adjacency

 The fromnode and to node of an arc
indicate its direction, and it helps
determining the polygons on its left
and right side.
 In the illustration above, polygon B is
on the left and polygon C is on the
right of the arc 4.
 Polygon A is outside the boundary of
the area covered by polygons B, C
and D. It is called the external or
universe polygon

Containment
 Geographic features cover
distinguishable area on the surface of
the earth.
 The polygons can be simple or they can
be complex with a hole or island in the
middle.
 In the illustration given below assume a
lake with an island in the middle.
 The lake actually has two boundaries,
one which defines its outer edge and the
other (island) which defines its inner

 The polygon D is made up of arc 5, 6
and 7.
 The 0 before the 7 indicates that the
arc 7 creates an island in the polygon.

 Polygons are represented as an
ordered list of arcs and not in terms of
X, Y coordinates. This is called
Polygon-Arc topology
 Since arcs define the boundary of
polygon, arc coordinates are stored
only once, thereby reducing the
amount of data and ensuring no
overlap of boundaries of the adjacent
polygons.

Polygon as a topological
feature

Topology Rules
 There are many topology rules can be
implemented in the geodatabase, that’s depends
upon the spatial relationships that any
organization maintains. Some topology rules
govern the relationships of features within a
given feature class, while others govern the
relationships between features in two different
feature classes or subtypes. Topology rules can
be defined between subtypes of features in one
or another feature class. Three Topology rules
are:
◦ Polygon rules
◦ Line rules
◦ Point rules

Polygon Rules
1. Must Be Larger Than Cluster Tolerance
2. Must Not Overlap
3. Must Not Have Gaps
4. Must Not Overlap With
5. Must Be Covered By Feature Class Of
6. Must Cover Each Other
7. Must Be Covered By
8. Boundary Must Be Covered By
9. Area Boundary Must Be Covered By
Boundary Of
10. Contains Point

Line Rules
1. Must Be Larger Than Cluster Tolerance
2. Must Not Overlap
3. Must Not Intersect
4. Must Be Covered By Feature Class Of
5. Must Not Overlap With
6. Must Be Covered By Boundary Of
7. Must Not Have Dangles
8. Must Not Have Pseudo Nodes
9. Must Not Self-Intersect
10. Must Be Single Part
11. Must Not Intersect Or Touch Interior With
12. Endpoint Must Be Covered By
13. Must Not Self-Overlap

Point Rules
1. Must Be Single Part
2. Must Be Disjoint
3. Must Be Covered By Boundary Of
4. Must Be Properly Inside
5. Must Be Covered By Endpoint Of
6. Point Must Be Covered By Line

Attribute Linking
 Most database design guidelines promote organizing
user database into multiple tables each focused on a
specific topic instead of one large table containing all
the necessary fields.
 Having multiple tables prevents duplicating
information in the database, because user store the
information only once in one table. When user need
information that isn't in the current table, user can link
the two tables together.
 For example, user might obtain data from other
departments in user organization, purchase
commercially available data, or download data from
the Internet. If this information is stored in a table,
such as a dBASE, INFO, or geodatabase table, user
can associate it with user geographic features and
display the data on user map.
 ArcGIS allows user to associate records in one table
with records in another table through a common field,
known as a key.

Joining the attributes from a
table
 Typically, user'll join a table of data to a layer
based on the value of a field that can be found
in both tables. The name of the field does not
have to be the same, but the data type has to
be the same; user join numbers to numbers,
strings to strings, and so on. User can perform
a join with either the Join Data dialog box,
accessed by right-clicking a layer in ArcMap, or
the Add Join tool.
 Suppose user have obtained data that
describes the percentage change in population
by country and user want to generate some
population growth maps based on this
information. As long as the population data is
stored in a table in user database and shares a
common field with user layer, user can join it to
user geographic features and use any of the
additional fields to symbolize, label, query, or

Attribute Linking
 Joins
 Relations
◦ One to one
◦ One to many
◦ Many to many

Spatial Join
 Match each feature to the closest
feature or features
 Match each feature to the feature
that it is within
 Match each feature to the feature or
features that it intersects

Joins versus
relates
 User'll want to join two tables when
the data in the tables has a one-to-one
or a many-to-one relationship.
 User'll want to relate two tables when
the data in the tables has a one-to-
many or many-to-many relationship.

ODBC
 Open Database Connectivity (ODBC) is an open
standard application programming interface (API)
that allows application programmers to access any
database. ODBC consists of four components,
working together to enable functions.
Components of ODBC
The four different components of ODBC are:
• Application: Processes and calls the ODBC
functions and submits the SQL statements;
• Driver manager: Loads drivers for each application;
• Driver: Handles ODBC function calls, and then
submits each SQL request to a data source;
• Data source: The data being accessed and its
database management system (DBMS) OS.

Connect to the database
 The following steps describe using the Database
Connection dialog box.
1. Expand Database Connections in the Catalog tree
in ArcGIS or ArcCatalog and double-click Add
Database Connection.
2. Choose SQL Server from the Database
Platform drop-down list.
3. Type the SQL Server instance name in
the Instance text box.
For example, if user are using a default SQL
Server instance, user can specify the instance name or the
IP address of the server in the Instance text box. If
specifying an IPV6 address, enclose the address in
brackets. For example, if the IPV6 address of the server is
2000:ab1:0:2:f333:c432:55f6:d7ee,
type [2000:ab1:0:2:f333:c432:55f6:d7ee] in
the Instance text box.

4. Choose the type of authentication to use when
connecting to the database: Database
authentication or Operating system
authentication.
If user choose Operating system authentication,
user do not need to type a user name and
password—the connection is made using the login
name and password used to log in to the operating
system.
If user choose Database authentication, user
must provide a valid database user name and
password in the User name and Password text
boxes, respectively. User names can be a
maximum of 30 characters.

5. Note:
Save user name and password must be
checked for connection files that provide
ArcGIS services with access to the database
Outside of ArcGIS, user can create user
names in SQL Server that contain special
characters
6. In the Database text box, type or choose
the name of the specific database user want
to connect to on the SQL Server.
In the following example, a connection is
made to the database, spatial data, on
the SQL
Server instance server1ss08r2 using

7. Click OK to connect.
A file is created
in <computer_name>Users<user_n
ame>AppDataRoamingESRIDeskto
p<release#>ArcGIS.

GPS
 GPS stands for Global Positioning System
by which anyone can always obtain the
position information anywhere in the world.
 BASIC STRUCTURE OF GPS
Three-block configuration

 The GPS system consists of three
segments:
1) The space segment: the GPS satellites
2) The control system, operated by
the U.S. military,
3) The user segment, which includes both
military and civilian users and their GPS
equipment.

 Space Segment:
 The space segment is the number of
satellites in the constellation. It
comprises of 29 satellites circling the
earth every 12 hours at 12,000 miles
in altitude.
 The function of the space segment is
utilized to route/navigation signals
and to store and retransmit the
route/navigation message sent by the
control segment.

 Control Segment:
 The control segment comprises of a master
control station and five monitor stations
outfitted with atomic clocks that are spread
around the globe.
 The five monitor stations monitor the GPS
satellite signals and then send that qualified
information to the master control station
where abnormalities are revised and sent
back to the GPS satellites through ground
antennas.
 Control segment also referred as monitor
station.

 User Segment:
The user segment comprises of the GPS
receiver, which receives the signals from
the GPS satellites and determine how far
away it is from each satellite.
Mainly this segment is used for the U.S
military, missile guidance systems, civilian
applications for GPS in almost every field.

GPS POSITIONING
 The working/operation of Global positioning system
is based on the ‘trilateration’ mathematical principle.
 The position is determined from the distance
measurements to satellites.
 From the figure, the four satellites are used to
determine the position of the receiver on the earth.
The target location is confirmed by the 4th satellite.
And three satellites are used to trace the location
place. A fourth satellite is used to confirm the target
location of each of those space vehicles.
 Global positioning system consists of satellite,
control station and monitor station and receiver.
 The GPS receiver takes the information from the
satellite and uses the method of triangulation to
determine a user’s exact position.

GPS POSITIONING
 GPS is used on some incidents in several
ways, such as:
 To determine position locations; for example,
you need to radio a helicopter pilot the
coordinates of your position location so the pilot
can pick you up.
 To navigate from one location to another; for
example, you need to travel from a lookout to
the fire perimeter.
 To create digitized maps; for example, you are
assigned to plot the fire perimeter and hot spots.
 To determine distance between two different
points.

Advantages of GPS
 GPS satellite based navigation system is
an important tool for military, civil and
commercial users
 Vehicle tracking systems GPS-based
navigation systems can provide us with
turn by turn directions
 Very high speed

Disadvantages of GPS
 GPS satellite signals are too weak when
compared to phone signals, so it doesn’t
work as well indoors, underwater, under
trees, etc.
 The highest accuracy requires line-of-
sight from the receiver to the satellite,
this is why GPS doesn’t work very well in
an urban environment.

GPS Working
 Firstly, the signal of time is sent from a
GPS satellite at a given point.
 Subsequently, the time difference
between GPS time and the point of time
clock which GPS receiver receives the
time signal will be calculated to generate
the distance from the receiver to the
satellite.

 Since navigation message consists of
25 frames, this would add up to the
message length of 12.5 minutes (30
seconds x 25=12.5 minutes).
 The GPS receiver requires 12.5
minutes to receive all the necessary
set of data, necessary condition for
positioning, when initial power
activation takes place.

POSITIONING ACCURACY
 Factors that trigger GPS position errors
 Ionosphere
The ionosphere is a portion of the upper atmosphere,
between the thermosphere and the exosphere. When GPS
signals pass through this layer, the propagation velocity of the
GPS signal goes slower, hence causing propagation error.
 Troposphere
The troposphere is the lowest portion of Earth's
atmosphere. Radio reflections caused by dry atmosphere and
water vapor within provoke GPS position error.
 Multipath propagation
GPS signal is not immune to reflection when it hits on
the ground, structures and many others. This phenomenon is
called multipath propagation, one of the causes of GPS
position errors.

DOP (Dilution Of Precision)
 DOP is a value that shows the degree of
degradation of the GPS positioning
accuracy. The smaller the value is, the
higher the positioning accuracy is. This
value depends upon the positions of the
GPS satellites tracked for positioning. If the
tracked satellites spread evenly over the
earth, the positioning accuracy would
become higher, and if the positions of
tracked satellites are disproportionate, the
positioning accuracy would become lower.

Signal Strength
 State of reception of GPS depends upon
the strength of GPS signals. The greater
the signal strength is, the more stable the
reception status is. Whereas the
reception status would become unstable
when the GPS signal became weaker,
due to obstacles or noise sources in the
vicinity of a GPS receiver.

Gis unit 3

More Related Content

What's hot (20)

Similar to Gis unit 3 (20)

More from sridevi5983 (12)

Recently uploaded (20)

Gis unit 3