GIS Exam Two

Select By Location

• Vector Analysis

• Select features from one layer based on their spatial association with another layer

  • Examples: Adjacency, Containment, Distance, Intersect

Adjacency

Features from one layer share a boundary with another

Containment

Features located within another feature

Distance

One feature is within a certain distance of another

Intersect

Features that fully or partially overlap another feature

• Contained objects also do this

Select by attribute/location do NOT

Create a new layer

Geoprocessing

When one layer has some sort of action performed to it and a new layer is created

Layer

One file of geographic data in GIS. Can refer to either vector or raster.

Two categories of geoprocessing

1. One layer analysis

2. Two+ layer analysis

One Layer Analysis

• Buffer

• Dissolve

Buffer

• Area of proximity that can be created around one or more objects

• Can be created around points, lines, polygons

• Can be used for spatial selection

  • features within a buffer or that intersect a buffer, etc.

Dissolve

• When adjacent polygons that have the same properties have their boundaries removed

• Merges polygons into a single, larger shape

• Ex:

  • counties in a state, congressional districts, land cover types

Two+ Layer Analysis

• Overlay geoprocessing operations

• When two or more things that share the same spatial boundaries are combined

• Union, intersect, identity, and symmetrical difference

Union

All features from both layers are combined together into a new layer. Acts as OR

Intersect

Only features that layers have in common are retained in the new layer. Acts as AND

Identity

• All features of the first layer are retained

• All features of the second layer where it intersects the first are also retained

• Order MATTERS

Symmetrical Difference

• Features of both layers are contained, except for what they have in common

• Opposite of intersect

• Acts as XOR

• holes where they intersect

Raster Resolution

• Based on cell length (one side ONLY)

• Cell size = length of one side of a cell in the real world

• Coarse vs fine resolution

  • small cell size = finer resolution

  • larger cell size = coarser resolution

Local Operations (Cell-By-Cell)

• 1 raster file

• All cells have the same operation done to them using the same value

  • Addition, subtraction, multiplication, division, sqaure root, squaring

Zonal Operations

• 2 raster file

  • 1 input, 1 zonal

• Minimum, maximum, average, sum

Map Algebra

• Similar to local operation

• At least two rasters stacked on top of each other

• Addition, subtraction, multiplication, division

GNSS

• Global Navigation Satellite System

• AKA GPS

• Has at least 24 satellites for global coverage

NAVSTAR GPS

United States GNSS

• stands for Navigation Satellite Timing and Ranging

GLONASS

Russian GNSS

Galileo

European Union GNSS

BeiDou

Chinese GNSS

Three Main Segments of GNSS

1. Satellite/space segment

2. Control segment

3. User segment

Satellite/Space segment of GNSS

• Consists of a constellation of satellites orbiting the earth

• Constellations: groups of 4 satellites in 6 orbital planes

L1

• Coarse Acquisition (C/A): Code for civilian use

• Precise (P): Code for military and civilian use

L2

• P Code

• P (Y) Code: Encrypted version of P code

Control Segment

• monitoring and control stations

• consist of master control stations and ground antennas

User Segment

• all GPS units that receive satellite signals

What information satellites transmitt

• time and location

Satellite Almanac

Contains orbit and location data for all satellites in a constellation

How to calculate distance

Distance = time x speed

Psuedorange

Distance between a satellite transmitting a signal and the receiver

3D Trilateration

Process used by satellites to find location on earth using distances

Sources of innacuracy

• selective availibility

• ephemeris error

• position dilution of precision (PDOP)

• atmospheric errors

• multipath errors

Selective Availability (SA)

• Intentionally introducing errors to make C/A less accurate for civilian use

• Two Types

  • Delta Error

    • incorrect timing info

  • Epsilon Error

    • incorrect satellite location info (ephemeris)

• Removed by US Govt in 2000

Ephemeris Error

• Occurs when satellite broadcasts incorrect location information

• Data is corrected and updated by a control station

Position Dilution of Precision Error (PDOP)

• Error due to positional location of satellites in the sky

• When satellites are too close to each other, it reduces their accuracy

Atmospheric Error

• Ionosphere

  • Charged particles alter speed of signal, causing inaccuracies in timing

• Troposphere

  • Water vapor delays signals

Multipath Error

• Occurs when the receiver has a poor view of the sky

• Objects blocking the signal (tree canopy)

• Objects reflecting the signal (buildings, water)

• Can’t be compensated for

Ways to improve accuracy

1. Differential GPS (DGPS)

2. Satellite-Based Augmentation Systems (SBAS)

Differential GPS

• Base station at known coordinates

• Base station broadcasts correction signal

• GPS receivers pick up signals from 4 signals plus the correction

• Example: NDGPS and CORS

Nationwide Differential GPS

• NDGPS

• Used by coast guard for ship navigation

• Example of DGPS

Continually Operating Reference Station

• CORS

• Operated by National Geodetic Survey

• Example of DGPS

Satellite-Based Augmentation Systems

• Base stations broadcast correction to SBAS satellite

• SBAS satellite broadcasts connection to GPS receiver

• Example: WAAS

Wide Area Augmentation System

• WAAS

• Developed by Federal Aviation Administration for aircraft positional info

• SBAS example

3 Categories of International Systems

1. Full GNSS

2. Regional Systems

3. Augmenting Systems

   • SBAS

Full GNSSs

1. NAVSTAR GPS

2. GLONASS

3. Galileo

4. BeiDou-2

Indian Regional Navigation Satellite System

• Only regional system we’ve learned about

• IRNSS

Augmented Systems

1. BeiDou-1

   • SBAS for BeiDou2

2. GAGAN

   • SBAS for IRNSS

3. QZSS

   • SBAS for Japan/Oceania

Applications of GPS

• military

• transportation

• wildlife/livestock management

• pet tracking

• precision agriculture

• entertainment