GIS week 2

Question 1

What is the difference between GPS and GNSS?

Answer 1

GPS (Global Positioning System) is a specific satellite navigation system owned by the US.

GNSS (Global Navigation Satellite Systems) is the generic term for all satellite positioning systems (e.g., GPS, Galileo, GLONASS).

Both are part of PNT services, providing Position, Navigation, and Timing.

Question 2

What is the origin and development timeline of GPS?

Answer 2

Owned/Run by: US Military

1st Satellite Launched: 1973

Fully Operational: 1995

Civilian Use: 1980s (with selective availability limiting accuracy)

GPS was initially for military use but gradually opened for civilian use in the 1980s.

Question 3

What are the key features of the GPS Space Segment?

Answer 3

Made up of at least 24 satellites (currently 31 in orbit).

Satellites have advanced atomic clocks for high-precision timing.

Improvements include better accuracy, signal strength, and quality.

Satellites have a 12-year design lifespan.

Satellites were launched between 2010–2016.

Question 4

What are the key components of the GPS Control Segment?

Answer 4

The Control Segment consists of:

Master Control Station (red star) – main operational hub (Schriever AFB, Colorado).

Alternate Master Control Station (yellow star).

Ground Antennas (green triangle) – transmit and receive data to/from satellites.

Air Force Monitor Stations (blue circles) – track satellite signals for accuracy.

AFSCN Remote Tracking Stations (yellow triangles).

NGA Monitor Stations (purple circles) – provide additional monitoring locations.

These elements are globally distributed across locations such as the US, UK, South Korea, Guam, Diego Garcia, and others.

Question 5

What are the functions of GPS Monitor Stations?

Answer 5

Track GPS satellites as they pass overhead.

Collect navigation signals, range/carrier measurements, and atmospheric data.

Feed observations to the master control station.

Use sophisticated GPS receivers.

Provide global coverage through 16 sites.

Question 6

What is the role of the GPS Master Control Station?

Answer 6

The Master Control Station:

Provides command and control of the GPS satellite constellation.

Uses data from global monitor stations to compute precise satellite locations.

Generates navigation messages for upload to satellites.

Monitors satellite broadcasts and system integrity to ensure health and accuracy.

Performs maintenance and anomaly resolution, including repositioning satellites.

Is backed up by a fully operational alternate master control station.

Question 7

What are the functions of GPS Ground Antennas?

Answer 7

Ground antennas:

Send commands, navigation data uploads, and processor program loads to satellites.

Collect telemetry (data from satellites).

Communicate via S-band signals and perform S-band ranging for anomaly resolution and early orbit support.

Consist of 4 dedicated GPS ground antennas + 7 Air Force Satellite Control Network (AFSCN) remote tracking stations.

Question 8

What is the User Segment in GPS, and what are examples of user devices?

Answer 8

The User Segment refers to devices that receive GPS signals and use them for positioning, navigation, and timing.
Examples of user devices include:

Handheld GPS receivers (e.g., Garmin eTrex)

GPS-enabled watches (e.g., Garmin sports watch)

In-vehicle navigation systems (e.g., car satnavs)

Survey-grade GPS units (e.g., Trimble devices used for professional surveying and mapping)

Marine and aviation navigation systems

Question 9

What are the five key steps in how GPS works?

Answer 9

1. Trilateration (not triangulation): Determining relative positions using distances (not angles) from at least 3 satellites.
2. GPS receiver measures distance using the travel time of radio signals.
3. GPS requires very accurate timing (from atomic clocks).
4. GPS needs to know exact satellite positions in space.
5. GPS must correct for atmospheric delays as signals pass through the atmosphere.

Question 10

What is the difference between triangulation and trilateration in GPS, and how is location determined?

Answer 10

Trilateration is the correct term used in GPS. It calculates position using distances from satellites, not angles.

Triangulation (less accurate term in this context) uses angles to determine position.

GPS receivers calculate position by measuring distances from at least three satellites, creating overlapping spheres.

The point where the spheres intersect (or overlap) is the precise location on Earth.

Question 11

How does a GPS receiver measure distance using time?

Answer 11

Satellites and receivers generate the same pseudo-random codes at the same time.

The receiver measures when a signal is received, but it also needs to know when it was sent.

The offset (difference) between the satellite’s and receiver’s codes indicates how far out of sync they are, which equals travel time.

Distance = Travel time × Speed of light.

Question 12

Why do GPS receivers not need atomic clocks?

Answer 12

Atomic clocks are expensive ($50K–$100K), so they’re not practical for regular GPS receivers.

In theory, only 3 satellites are needed for trilateration.

The 4th satellite is used to detect and correct timing errors.

By comparing the 4th satellite’s position with the other 3, GPS can calculate the timing correction without needing an atomic clock in the receiver.

Question 13

What are the main sources of GPS errors, ranked from highest to lowest impact?

Answer 13

1. Timing (clock in source) – Errors from satellite atomic clocks.
2. Upper atmosphere (ionosphere) – Delays in signal as it passes through.
3. Timing (receiver) – Inaccuracy of receiver’s internal clock.
4. Satellite orbit – Imperfect knowledge of exact satellite positions.
5. Lower atmosphere – Delays caused by the troposphere, tropopause, and stratosphere.
6. Multipath – Signal reflections off surfaces like buildings or mountains.

Question 14

What is Dilution of Precision (DOP) in GPS, and how does satellite geometry affect it?

Answer 14

DOP measures the effect of satellite geometry on the accuracy of GPS positioning.

Types include:

HDOP (Horizontal DOP): accuracy in horizontal position.

VDOP (Vertical DOP): accuracy in vertical position.

PDOP (Position DOP): combined effect on position accuracy.

A larger DOP value indicates poorer accuracy due to poor satellite geometry (satellites close together).

A smaller DOP value indicates better accuracy from good satellite geometry (widely spaced satellites).

Related to the volume of the pyramid formed by the satellites and receiver.

Question 15

What is Geometric Dilution of Precision (GDOP) in GPS, and how does satellite geometry affect it?

Answer 15

GDOP refers to the effect of satellite geometry on the precision of GPS measurements.

Poor satellite geometry (e.g., satellites clustered together) results in high GDOP (lower accuracy).

Good satellite geometry (e.g., satellites widely spaced) gives low GDOP (higher accuracy).

In the diagrams:

A shows poor geometry (larger uncertainty zones).

B and C show better geometry with reduced error areas (overlapping circles reduce uncertainty).

Question 16

How does Differential GPS (DGPS) improve accuracy compared to standard GPS?

Answer 16

Fixed reference receiver (at a known, accurately surveyed location) receives the same GPS signals as the roving receiver.

It uses its known position to calculate the expected travel time of GPS signals.

The reference receiver compares the expected vs. actual signal travel time to identify errors.

It then transmits RTCM corrections to the roving receiver, helping correct its timing and improve positional accuracy.

Question 17

How accurate are different types of GPS devices?

Answer 17

Your phone: ~10m+ accuracy

Consumer grade GPS: ~10m accuracy

Consumer grade with WAAS: 1–3m accuracy

Trimble R1 GNSS:

<1.0m real time

<50cm post-processed

Trimble R2 GNSS:

<1.0cm accuracy

Question 18

What is the BeiDou Navigation Satellite System (BDS), and what are its key features?

Answer 18

BeiDou (BDS) is a regional GNSS owned and operated by the People’s Republic of China.

BDS was previously called Compass.

China aims to expand BDS for global coverage, with a goal of 35 satellites by 2020.

Question 19

What is the Galileo system, and what are its key features?

Answer 19

Galileo is an independent high-precision GNSS operated by the EU European Space Agency (ESA) through the European GNSS Agency (GSA).

Uses 30 Medium Earth Orbit (MEO) satellites, including 6 spares.

Headquarters: Prague, Czech Republic.

Ground operation centres: Italy and Germany.

Timeline:

Went live in 2016/2018.

By 2020: 26 satellites launched (22 operational).

By end of 2021: 24 active, with upgrades planned for 2025.

Accuracy: less than 1 metre, down to 1.6 cm.

Question 20

What is GLONASS, and what are its key features?

Answer 20

GLONASS stands for Globalnaya Navigazionnaya Sputnikovaya Sistema (Global Navigation Satellite System).

It is operated by the Russian Federation.

The fully operational system consists of 24+ satellites providing global navigation coverage.

Question 21

What is IRNSS (NavIC), and what are its key features?

Answer 21

IRNSS: Indian Regional Navigation Satellite System, a regional GNSS operated by the Government of India.

NavIC: Stands for Navigation Indian Constellation (renamed in 2016).

Coverage: Indian region + 1500 km around the mainland.

Satellites:

7 satellites operational by 2018.

Plans to expand to 11 satellites.

NavIC usage:

Compulsory for commercial vehicles in India.

Available on consumer mobile phones from 2020.

Question 22

What is QZSS (Quasi-Zenith Satellite System), and what are its key features?

Answer 22

QZSS is a regional GNSS owned by the Government of Japan, operated by QZS System Service Inc. (QSS).

Also known as Michibiki.

Complements GPS to improve coverage in East Asia-Oceania.

Operational status:

4 satellites as of Nov 1st, 2018.

Plan to expand to 7 satellites for autonomous capability by 2023.

Question 23

What is the nature of spatial data?

Answer 23

Spatial data are usually observational rather than experimental.

They capture the complexity of the real world in finite form, through a process of conceptualisation and representation.

Spatial data record locations based on a georeferencing system.

Question 24

What is the object view in GIS?

Answer 24

The object view sees the world as populated by individual objects.

These objects are points, lines, and areas (features).

They are exactly located, have well-defined boundaries, and are countable.

Example: A road map showing highways, buildings, rivers as separate objects.

Question 25

What is the field view in GIS?

Answer 25

The field view represents the world as continuous surfaces. Attribute values (like elevation, temperature, or rainfall) can be defined at every location across the surface. It’s ideal for representing gradual variations in space, such as terrain, soil types, or vegetation density.

Question 26

What is a Digital Elevation Model (DEM)?

Answer 26

A 3D representation of a terrain surface. Shows bare-earth elevation: excludes vegetation, buildings, and man-made structures. Can be raster-based (grid of squares) or vector-based (TIN data).

Question 27

What is a Digital Terrain Model (DTM)?

Answer 27

A DEM plus extra surface features like slope, curvature, and drainage structures. Provides more detailed terrain information. Often used interchangeably with DEM, but more specific. DTM = DEM + Extra surface details (slope, curvature, drainage).

Question 28

What do the light and dark areas represent in a DTM?

Answer 28

Light areas: High elevations (hills, mountains) Dark areas: Low elevations (valleys, depressions)

Question 29

How are DTMs usually represented?

Answer 29

As grayscale raster images where brightness indicates elevation.

Question 30

What is the difference between a DEM and DTM?

Answer 30

A DTM is a DEM plus extra surface features, such as slope, curvature, and drainage.

Question 31

What is relief in topography?

Answer 31

The difference in elevation between the highest and lowest points in an area.

Question 32

What is a topographic map?

Answer 32

A map that represents the shape of the land surface using contour lines.

Question 33

What do contour lines show on a topographic map?

Answer 33

Lines of equal elevation. They connect points at the same height above sea level.

Question 34

How do you identify steep slopes on a contour map?

Answer 34

Closely spaced contour lines indicate steep slopes.

Question 35

How do you identify gentle slopes on a contour map?

Answer 35

Widely spaced contour lines indicate gentle slopes.

Question 36

What is elevation?

Answer 36

The height of a point above sea level.

Question 37

What does 3D visualisation of terrain help to show?

Answer 37

It highlights features like hills, valleys, and slopes, providing a more intuitive understanding of topography.

Question 38

Why is 3D visualisation useful in geography?

Answer 38

It helps in visualising terrain elevation and shape, useful for planning, analysis, and decision-making.

Question 39

What is Lantern Pike in this context?

Answer 39

A specific geographic feature identified on the 3D model as an example of terrain analysis.

Question 40

How are high and low elevations represented in 3D models?

Answer 40

Usually, higher elevations are shown as peaks, and lower elevations as depressions or valleys in the model.

Question 41

What is "draping" in the context of mapping?

Answer 41

Draping involves overlaying 2D map layers (like roads, rivers, and labels) onto a 3D terrain surface.

Question 42

Why is draping layers on a 3D surface useful?

Answer 42

It helps combine detailed map information with 3D terrain features, improving spatial understanding and navigation.

Question 43

What kind of data is typically "draped" over a 3D surface?

Answer 43

Map features like roads, contours, place names, land use patterns, and points of interest.

Question 44

What effect does draping have on a 3D terrain model?

Answer 44

It makes the model more informative and visually rich, combining spatial elevation with geographic detail.

Question 45

What does the 3D surface provide in a draped map?

Answer 45

The shape and elevation of the land (hills, valleys, etc.).

Question 46

What is the advantage of draping over a flat map?

Answer 46

It allows for a more realistic and interactive visualization of both terrain and geographic features.

Question 47

What type of information can be draped onto a 3D surface besides roads and labels?

Answer 47

Thematic layers like geological formations, soil types, land use categories, and vegetation zones.

Question 48

What do different colours on a 3D draped surface represent?

Answer 48

They indicate different categories or classes in the thematic data (e.g., rock types, land use zones).

Question 49

How does draping thematic data on a 3D surface help in analysis?

Answer 49

It visually links surface features (elevation, slope) with thematic information, helping in interpretation of complex landscapes.

Question 50

Why is a 3D perspective important in geology or geography?

Answer 50

It reveals how geological or thematic patterns relate to the underlying topography, aiding in interpretation and decision-making.

Question 51

What is the main advantage of 3D draping in mapping?

Answer 51

Combines spatial (elevation) and attribute (thematic) data in a single visual model, enhancing understanding of the landscape.

Question 52

What does TIN stand for in GIS?

Answer 52

Triangulated Irregular Network.

Question 53

What type of data model is TIN?

Answer 53

Vector-based model for surface morphology.

Question 54

How is the surface represented in TIN?

Answer 54

As a contiguous layer of triangles.

Question 55

What is the geometric property of each triangle in a TIN?

Answer 55

Each triangle is a plane with a constant gradient.

Question 56

What are the points (nodes) in TIN defined by?

Answer 56

3D coordinates (x, y, z).

Question 57

Why is TIN useful for surface modeling?

Answer 57

It can model complex surfaces with varying detail, using triangles that better match irregular terrain than a grid.

Question 58

How does a Triangulated Irregular Network (TIN) represent surface morphology in GIS?

Answer 58

By using contiguous triangles with constant gradients, each defined by 3D coordinates (x, y, z), allowing a more accurate surface model than a grid.

Question 59

What does a Digital Elevation Model (DEM) using a Triangulated Irregular Network (TIN) and 10m contours represent?

Answer 59

It represents a 3D terrain model where elevation changes are shown through color-coded bands at 10-meter intervals, capturing variations in terrain height across the landscape.

Question 60

What does the term map scale mean, and how does it affect the level of detail on a map?

Answer 60

Map scale refers to the ratio between distances on a map and their corresponding distances on the ground. A smaller map scale shows a larger area with less detail, while a larger map scale shows a smaller area with more detail.

Question 61

What is the purpose of cartographic generalisation, and what are its key techniques?

Answer 61

Cartographic generalisation transforms real-world features into simplified map representations. Techniques include: -selection (choosing key features) -simplification (removing detail) -exaggeration (emphasising important characteristics) -combination (merging similar features) -displacement (adjusting positions for clarity)

Question 62

What shape is the Earth, and how is it mathematically described?

Answer 62

The Earth is an oblate spheroid—slightly flattened at the poles and bulging at the equator. This is described by a semi-major axis (a), a semi-minor axis (b), and a flattening factor (f) given by: f= a-b/a ​

Question 63

What are Earth's major spheroids, and how do they differ?

Answer 63

Earth's major spheroids (e.g., WGS 1984, GRS 80, Airy 1830) vary in their semi-major and semi-minor axes and flattening factors. These variations affect map accuracy and satellite positioning.

Question 64

How does the geographic coordinate system locate positions on Earth?

Answer 64

It uses a grid of latitude (parallel lines) and longitude (meridian lines) to define locations in degrees. For example, P(40°E, 45°N) represents a specific point on the Earth's surface.

Question 65

What are the main types of map projections used in cartography?

Answer 65

The main types of map projections are cylindrical, conic, and azimuthal. Each projection translates the Earth's curved surface onto a flat plane in a different way.

Question 66

What are the differences between secant, normal, transverse, and oblique map projections?

Answer 66

Secant: A projection surface that cuts through the globe. Normal: The standard orientation of the projection. Transverse: The projection surface is rotated 90°. Oblique: The projection surface is tilted at an angle.

Question 67

How does a Secant Mercator projection adjust the Earth's surface to reduce distortion?

Answer 67

A Secant Mercator projection introduces two standard parallels where the scale is accurate. Between and beyond these lines, scale increases or decreases, helping to distribute distortion across the map.

Question 68

How does the UTM system divide the Earth for mapping purposes?

Answer 68

The UTM system divides the Earth into 60 zones, each 6° wide in longitude, providing a systematic way to project the Earth's curved surface onto a flat map with minimal distortion within each zone.

Question 69

What is a datum in geospatial contexts, and how does it affect coordinates?

Answer 69

A datum is a reference system for measuring locations on Earth. It defines the size and shape of the Earth and the origin and orientation of coordinate systems. Local datums are accurate for specific regions, while global datums (like WGS84) provide a global framework.

Question 70

What is the vector data model, and what are its key components?

Answer 70

The vector data model represents spatial features as points, lines, and polygons. These features are defined by coordinates and have attributes that describe their characteristics, making them ideal for precise location-based data like roads, rivers, and land parcels.

Question 71

How are geographic features represented in the vector data model?

Answer 71

The vector data model represents features as points, lines (polylines), or polygons, each defined by coordinate pairs (x, y). For example, a house is a point, a road is a line, and a lake is a polygon.

Question 72

How does the raster data model represent geographic features?

Answer 72

The raster data model divides space into a grid of cells, where each cell has a value representing a feature (e.g., land use or elevation). It simplifies spatial representation into a matrix of uniform cells.

Question 73

What are the key elements of a raster data structure?

Answer 73

A raster is defined by: Cells: Smallest units, each with a value. Cell value: Attribute of the feature at that location. Cell coordinates: The position (row, column) within the grid. Cell size: Resolution of each cell. UTM coordinates: Real-world coordinates for spatial reference.

Question 74

How does cell size affect raster representation?

Answer 74

Smaller cell sizes provide finer detail and accuracy, while larger cells simplify the data but lose detail. The choice of cell size impacts spatial resolution.

Question 75

What are the main types of spatial databases?

Answer 75

Spatial databases can be: Relational databases Geo-relational databases Object-relational databases

Question 76

What defines a relational database?

Answer 76

A relational database is a collection of interrelated tables, where each table is a relation. Tables are connected using primary keys and foreign keys.

Question 77

What is a geo-relational database in GIS?

Answer 77

A geo-relational database separates spatial geometry (like points, lines, and polygons) from attribute data (like names and values) but links them using a common identifier, allowing independent feature layers to occupy the same geographic space.