CRS in GIS. Cordinate Reference System

In the context of Geographic Information Systems (GIS), CRS stands for Coordinate Reference System. A Coordinate Reference System is a framework used to define and represent locations on the Earth's surface in a consistent and standardized manner.

The Earth is a three-dimensional object, and in order to accurately represent locations on its surface, we need a system that defines how coordinates are measured and referenced. A CRS provides a set of rules, parameters, and mathematical formulas to define the coordinates of points on the Earth's surface.

There are two main types of CRSs used in GIS: Geographic CRS and Projected CRS.

1. Geographic CRS: A Geographic CRS, also known as a geodetic CRS, uses latitude and longitude to define locations on the Earth's surface. Latitude measures the distance north or south of the Equator, while longitude measures the distance east or west of a reference meridian, typically the Prime Meridian (0 degrees longitude) that passes through Greenwich, London. Geographic CRSs are based on a spherical or ellipsoidal model of the Earth, such as the World Geodetic System 1984 (WGS84) or the North American Datum 1983 (NAD83).

2. Projected CRS: A Projected CRS, also known as a Cartesian CRS, uses a two-dimensional Cartesian coordinate system to represent locations on a flat surface, such as a paper map or a computer screen. Projected CRSs are derived from Geographic CRSs by applying mathematical transformations to convert the spherical or ellipsoidal coordinates into planar (x, y) coordinates. The transformation involves processes like flattening, scaling, and distortion correction. Projected CRSs are commonly used for measuring distances, areas, and directions accurately on maps. Examples of projected CRSs include the Universal Transverse Mercator (UTM) and the Lambert Conformal Conic (LCC) systems.

CRSs are essential in GIS because they ensure that geographic data from different sources can be accurately integrated and analyzed. When working with spatial data, it is crucial to use the correct CRS to avoid distortions, errors, and inconsistencies. GIS software allows users to define and assign CRSs to their datasets, ensuring proper alignment and accurate analysis.

It's worth noting that there are many different CRSs available, each suitable for specific regions, purposes, or map projections. Choosing the appropriate CRS depends on the geographic extent of your study area, the level of accuracy required, and the intended use of the data.

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...

Landsat 8 Band designation and Band Combination.

Landsat 8 Band designation and Band Combination. Landsat 8-9 Operational Land Imager (OLI) and Thermal Infrared Sensor (TIRS) Bands Wavelength (micrometers) Resolution (meters) Band 1 - Coastal aerosol 0.43-0.45 30 Band 2 - Blue 0.45-0.51 30 Band 3 - Green 0.53-0.59 30 Band 4 - Red 0.64-0.67 30 Band 5 - Near Infrared (NIR) 0.85-0.88 30 Band 6 - SWIR 1 1.57-1.65 30 Band 7 - SWIR 2 2.11-2.29 30 Band 8 - Panchromatic 0.50-0.68 15 Band 9 - Cirrus 1.36-1.38 30 Band 10 - Thermal Infrared (TIRS) 1 10.6-11.19 100 Band 11 - Thermal Infrared (TIRS) 2 11.50-12.51 100 Vineesh V Assistant Professor of Geography, Directorate of Education, Government of Kerala. https://www.facebook.com/Applied.Geography http://geogisgeo.blogspot.com

Landsat band composition

Short-Wave Infrared (7, 6 4) The short-wave infrared band combination uses SWIR-2 (7), SWIR-1 (6), and red (4). This composite displays vegetation in shades of green. While darker shades of green indicate denser vegetation, sparse vegetation has lighter shades. Urban areas are blue and soils have various shades of brown. Agriculture (6, 5, 2) This band combination uses SWIR-1 (6), near-infrared (5), and blue (2). It's commonly used for crop monitoring because of the use of short-wave and near-infrared. Healthy vegetation appears dark green. But bare earth has a magenta hue. Geology (7, 6, 2) The geology band combination uses SWIR-2 (7), SWIR-1 (6), and blue (2). This band combination is particularly useful for identifying geological formations, lithology features, and faults. Bathymetric (4, 3, 1) The bathymetric band combination (4,3,1) uses the red (4), green (3), and coastal bands to peak into water. The coastal band is useful in coastal, bathymetric, and aerosol studies because...

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...

Vineesh V, Geography

Search This Blog