Geographic and Projected Coordinate Systems

Geographic Coordinate System (GCS)

A Geographic Coordinate System (GCS) uses a three-dimensional spherical surface to define locations on Earth. It is based on a datum, an ellipsoid, and angular units (latitude and longitude).

Key Components of GCS:

Latitude (ϕ): Measures the north-south position relative to the Equator (0°). It ranges from 90°N to 90°S.
Longitude (λ): Measures the east-west position relative to the Prime Meridian (0°). It ranges from 180°E to 180°W.
Datum: Defines the reference ellipsoid and origin point.
Ellipsoid: An approximation of the Earth's shape.
Geoid: A model of Earth's gravitational surface.

Example of a Geographic Coordinate System:

WGS 84 (World Geodetic System 1984) – Used in GPS and global mapping applications.

Advantages of GCS:

Accurately represents global locations.
Commonly used for spatial data storage and sharing.

Disadvantages of GCS:

Distances and areas are distorted because the Earth is not a perfect sphere.
Not ideal for detailed, local mapping due to distortions in scale.

Projected Coordinate System (PCS)

A Projected Coordinate System (PCS) is a two-dimensional representation of Earth's surface created by mathematically transforming the curved surface of the Earth onto a flat plane.

Key Components of PCS:

Projection Type: Defines how the Earth's surface is transformed.
X, Y Coordinates: Express positions in meters or feet instead of latitude and longitude.
Origin and Datum: Ensures projection accuracy.

Types of Projections in PCS:

Conformal Projection: Preserves shape but distorts area (e.g., Mercator projection).
Equal-Area Projection: Maintains area but distorts shape (e.g., Albers Equal-Area).
Equidistant Projection: Preserves distances along specific lines (e.g., Plate Carrée).
Azimuthal Projection: Maintains direction but distorts distance (e.g., Lambert Azimuthal Equal Area).

Example of a Projected Coordinate System:

UTM (Universal Transverse Mercator): Divides the world into 60 zones, each 6° wide.
State Plane Coordinate System (SPCS): Used for high-accuracy local mapping in the U.S.

Advantages of PCS:

Preserves distances and areas, making it ideal for local and regional mapping.
Allows for easier measurements and calculations in GIS.

Disadvantages of PCS:

Distortion increases when used for large-scale or global mapping.
Different projections may be required for different regions.

Datums in GIS

A datum is a mathematical model that defines the size, shape, and orientation of the Earth for mapping and surveying purposes.

Types of Datums:

A. Geodetic Datums (Horizontal Datums)

Used for positioning locations on the Earth's surface.

Global Datums:
- WGS 84: Used in GPS and most global applications.
- ITRF (International Terrestrial Reference Frame): Used for scientific measurements.
Local Datums:
- NAD 27 (North American Datum 1927): Based on Clarke 1866 Ellipsoid, used in North America.
- NAD 83 (North American Datum 1983): Updated to align with modern satellite data.

B. Vertical Datums

Used for measuring elevations and depths.

Geoid-Based:
- EGM96 (Earth Gravitational Model 1996): Used for global height measurements.
- NAVD88 (North American Vertical Datum 1988): Used in North America.
Ellipsoid-Based:
- WGS 84 Ellipsoid Height: Used in GPS applications.

Example:

A point in New York City might have different coordinates in WGS 84 versus NAD 83 due to differences in datum definitions.

Ellipsoid and Geoid in GIS

These are models of Earth's shape used in geodesy and GIS.

Ellipsoid

A mathematically defined smooth surface that approximates the shape of the Earth.
Used in map projections and coordinate systems.
Example: GRS 80 (Geodetic Reference System 1980), WGS 84.

Geoid

Represents the actual shape of the Earth based on gravity measurements.
More accurate for elevation measurements than ellipsoids.
Example: EGM2008 (Earth Gravitational Model 2008).

Key Differences:

Feature	Ellipsoid	Geoid
Definition	Smooth mathematical model of Earth	Realistic Earth shape based on gravity
Purpose	Used for mapping and coordinate systems	Used for measuring precise elevations
Example	WGS 84, GRS 80	EGM96, NAVD88

Example in GIS:

GPS uses an ellipsoid model (WGS 84) for positioning.
Elevation data often references the geoid (EGM96) for height measurements.

GCS (Latitude-Longitude) is best for global positioning but introduces distortions.
PCS (Projected X-Y coordinates) is ideal for precise local measurements.
Datums define how coordinates align with Earth's shape.
Ellipsoids approximate Earth's smooth shape, while geoids account for gravitational variations.

Practical Applications in GIS:

GPS Navigation: Uses WGS 84 for positioning.
Land Surveys: Use local datums like NAD 83 for high-accuracy mapping.
Elevation Mapping: Uses geoid-based vertical datums for terrain analysis.
Urban Planning: Uses UTM projections for detailed city mapping.

Comments

Accuracy Assessment

Accuracy assessment is the process of checking how correct your classified satellite image is . 👉 After supervised classification, the satellite image is divided into classes like: Water Forest Agriculture Built-up land Barren land But classification is done using computer algorithms, so some areas may be wrongly classified . 👉 Accuracy assessment helps to answer this question: ✔ "How much of my classified map is correct compared to real ground conditions?" Goal The main goal is to: Measure reliability of classified maps Identify classification errors Improve classification results Provide scientific validity to research 👉 Without accuracy assessment, a classified map is not considered scientifically reliable . Reference Data (Ground Truth Data) Reference data is real-world information used to check classification accuracy. It can be collected from: ✔ Field survey using GPS ✔ High-resolution satellite images (Google Earth etc.) ✔ Existing maps or survey reports 🧭 Exampl...

Change Detection

Change detection is the process of finding differences on the Earth's surface over time by comparing satellite images of the same area taken on different dates . After supervised classification , two classified maps (e.g., Year-1 and Year-2) are compared to identify land use / land cover changes . Goal To detect where , what , and how much change has occurred To monitor urban growth, deforestation, floods, agriculture, etc. Basic Concept Forest → Forest = No change Forest → Urban = Change detected Key Terminologies Multi-temporal images : Images of the same area at different times Post-classification comparison : Comparing two classified maps Change matrix : Table showing class-to-class change Change / No-change : Whether land cover remains same or different Main Methods Post-classification comparison – Most common and easy Image differencing – Subtract pixel values Image ratioing – Divide pixel values Deep learning methods – Advanced AI-based detection Examples Agricult...

Development and scope of Environmental Geography and Recent concepts in environmental Geography

Environmental Geography studies the relationship between humans and nature in a spatial (place-based) way. It combines Physical Geography (natural processes) and Human Geography (human activities). A. Early Stage 🔹 Environmental Determinism Concept: Nature controls human life. Meaning: Climate, landforms, and soil decide how people live. Example: People in deserts (like Sahara Desert) live differently from people in fertile river valleys. 🔹 Possibilism Concept: Humans can modify nature. Meaning: Environment gives options, but humans make choices. Example: In dry areas like Rajasthan, people use irrigation to grow crops. 👉 In this stage, geography was mostly descriptive (explaining what exists). B. Evolution Stage (Mid-20th Century) Environmental problems increased due to: Industrialization Urbanization Deforestation Pollution Geographers started studying: Environmental degradation Resource management Human impact on ecosystems The field became analytical and problem-solving...

Geographic phenomena fields objects boundaries.

In geography, geographic phenomena refer to features or processes that can be observed and studied on Earth's surface. These phenomena can be classified into three main categories: fields , objects , and boundaries . Each category has distinct characteristics, representations, and applications in Geographic Information Systems (GIS). 1. Fields A field represents continuous, spatially varying data where a value is present at every location within the study area. It describes conditions that exist across a geographic area. Characteristics : Continuity : Fields have no discrete boundaries; the data is continuous. Gradual Variability : The values of a field change gradually across space. Representation : Typically modeled using raster data in GIS, where a grid structure assigns a value (e.g., temperature or elevation) to each cell. Examples : Temperature Map : Shows temperature variation across a region. Rainfall Distribution : Displays rainfall levels over a large g...

History of GIS

1. 1832 - Early Spatial Analysis in Epidemiology: - Charles Picquet creates a map in Paris detailing cholera deaths per 1,000 inhabitants. - Utilizes halftone color gradients for visual representation. 2. 1854 - John Snow's Cholera Outbreak Analysis: - Epidemiologist John Snow identifies cholera outbreak source in London using spatial analysis. - Maps casualties' residences and nearby water sources to pinpoint the outbreak's origin. 3. Early 20th Century - Photozincography and Layered Mapping: - Photozincography development allows maps to be split into layers for vegetation, water, etc. - Introduction of layers, later a key feature in GIS, for separate printing plates. 4. Mid-20th Century - Computer Facilitation of Cartography: - Waldo Tobler's 1959 publication details using computers for cartography. - Computer hardware development, driven by nuclear weapon research, leads to broader mapping applications by early 1960s. 5. 1960 - Canada Geograph...

Vineesh V, Geography

Search This Blog