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Earth observation basics
Earth observation basics

Martin Wieser

Terms

CRS

Coordinate Reference System: In the field of Geographical Information Systems (GIS), a CRS is a coordinate-based local, regional, or global system used to locate geographical entities.

A Coordinate Reference System (CRS) is used to locate geographical entities. There are many types of CRS used in geospatial mapping and GIS (Geographic Information Systems). They can be broadly divided into two types: geographic coordinate systems and projected coordinate systems.

1. **Geographic Coordinate Systems (GCS)**: These are based on a 3D model of the earth, but they use 2D coordinates to identify locations. The most common example of this is the Latitude-Longitude system, which is based on degrees of arc. The reference point (0,0) is typically the intersection of the Prime Meridian and the Equator.

2. **Projected Coordinate Systems (PCS)**: These are based on a plane (2D surface), and therefore, are more suitable for smaller areas like a specific country or region. A PCS uses linear units (like meters or feet) which makes it very useful for tasks requiring accurate distance and area measurements. Examples include the Universal Transverse Mercator (UTM) and the State Plane Coordinate System (SPCS) in the U.S.

Each of these systems comes in various versions which can have different parameters such as the datum, units of measure, and projection method. The datum describes the position of the surface relative to the center of the earth, the projection method transforms the 3D surface to a 2D plane, and the units of measure are usually in meters or feet.

In addition, there are also **Vertical Coordinate Systems (VCS)** used to record height or depth relative to a standard baseline, usually mean sea level.

Different systems are suitable for different purposes, and the choice depends on factors such as the area covered by the map and the specific requirements of the task at hand.

WGS84

WGS84 (World Geodetic System 1984) is a geodetic system that is used in geography, geology, navigation, mapping, and other fields that require a standardized coordinate system for the Earth. This standard is extensively used in GPS (Global Positioning System) technology.

The system was developed by the United States Department of Defense and has been in use since 1984. It defines several elements:

1. The shape and size of the Earth, referred to as the ellipsoid, is a mathematical model of the earth's shape, which is essentially a sphere slightly squashed at the poles and bulging at the equator.

2. The Earth's gravitational constant, including mass and the angular velocity of Earth.

3. The reference frame for latitude, longitude, and elevation. In WGS84, coordinates are expressed as a geographic latitude, longitude, and (optionally) elevation, where latitude and longitude are specified in decimal degrees and elevation is specified in meters.

4. A realization of the reference system, through a network of precisely surveyed points on the Earth's surface, from which a reference frame is calculated.

5. The Geoid, a model of global mean sea level that is used to accurately measure elevation.

The WGS84 coordinate system is used as the base for many maps and GPS systems worldwide, making it a fundamental tool for global navigation and mapping systems.

UTM

UTM stands for Universal Transverse Mercator. It is a coordinate system commonly used for mapping and navigation purposes. The UTM system divides the Earth into multiple zones, each spanning 6 degrees of longitude. The system provides a grid-based reference system that allows locations to be specified with a unique combination of zone, easting, and northing coordinates.

In the UTM system, the Earth's surface is projected onto a flat grid, resulting in distortion. However, this distortion is minimized within each individual zone, making it suitable for accurate measurements and calculations within a specific area. The UTM grid is commonly used for a variety of applications, such as surveying, mapping, GPS navigation, and military operations.

UTM coordinates are typically expressed as a pair of values: the easting and northing. The easting represents the horizontal position east of a designated reference point, while the northing represents the vertical position north of the equator or a designated reference point within the zone.

For example, a UTM coordinate might look like 32U 123456 987654, where "32U" refers to the UTM zone (Zone 32 in the northern hemisphere), and "123456" and "987654" represent the easting and northing values, respectively.

EPSG

EPSG:4326 is a coordinate reference system (CRS) used in geographic information systems (GIS), databases, and spatial analysis. This code refers to a specific CRS defined by the European Petroleum Survey Group (EPSG).

In EPSG:4326, coordinates are expressed as latitude and longitude values, in that order, with both values measured in decimal degrees. The coordinate system is based on the WGS84 datum, which is the standard for GPS technology and is used worldwide.

Here's a breakdown of what EPSG:4326 refers to:

- EPSG: The European Petroleum Survey Group, the organization that maintains the database of coordinate system information.

- 4326: The specific ID number of the coordinate reference system in the EPSG database.

So, if you see a geographic dataset labeled as being in EPSG:4326, it means that the data's spatial reference is in a latitude/longitude format based on the WGS84 datum.

VHR

very-high-resolution image

NDVI

Normalized Difference Vegetation Index

learn

more⁠

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change detection

deep learning

KPCA

kernel principal component analysis

https://towardsdatascience.com/kernel-pca-vs-pca-vs-ica-in-tensorflow-sklearn-60e17eb15a64

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K-means

https://en.wikipedia.org/wiki/K-means_clustering

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GDAL

Geospatial Data Abstraction Library

Tiles

Tool for Satellite

tiles⁠

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Data Collections

Sentinel tiles

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download⁠

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SentinelHub data

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collection⁠

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Corine Landcover

⁠

download⁠

and

here⁠

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Coperincus Landcover

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download⁠

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MS landcover

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download⁠

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Useful Links

Microsoft Planetary Computer

more⁠

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Sentinel2 in a serverless Lambda on AWS

more⁠

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AWS

resources⁠

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Tools

Google Earth Engine Pro

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download⁠

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Earth observation basicsEarth observation basics

Terms

CRS

WGS84

UTM

EPSG

VHR

NDVI

Data Collections

Useful Links

Tools

Earth observation basics
Earth observation basics