Converting coordinates from one CRS to another CRS
Coordinate conversion means transforming a point from one coordinate system (CRS) to another.
Two types:
1.Transformation (datum change, e.g., WGS84 → Indian 1954)
2.Projection (lat/lon → grid in meters, e.g., WGS84 → UTM)
Often both happen together.
What is CRS (Coordinate Reference System)?
A CRS defines how locations on Earth are described.
A CRS specifies:
•What shape of Earth you are using (datum/ellipsoid)
•How coordinates are measured (lat/lon, meters, etc.)
•What projection (if any) is used
•The area of validity
Types of CRS:
✔ Geographic CRS
•Uses latitude & longitude
•Units: degrees
•Example: EPSG:4326 (WGS84)
✔ Projected CRS
•Uses X, Y coordinates in meters
•Derived using a map projection
•Example: EPSG:32643 (WGS84 / UTM Zone 43N)
✔ Vertical CRS
•Heights: ellipsoidal or orthometric
•Example: EGM96 geoid height
In simple words, CRS = How you define and measure positions on Earth.
EPSG stands for European Petroleum Survey Group, now maintained by the OGP / IOGP Surveying & Positioning Committee.
EPSG maintains the world’s official database of:
•Coordinate Reference Systems (CRS)
•Map projections
•Datums
•Transformation methods
•Geoid models
Each CRS in the database has a unique EPSG code. For all EPSG codes, visit https://www.epsg.io
What is EPSG:4326?
It is ....
•WGS84 datum
•Latitude & Longitude in degrees
•Ellipsoidal coordinate system
•Used by GPS, Google Earth, mobile phones, etc.
What is PROJ4?
Proj4 (now simply called PROJ) is an open-source library used worldwide for:
•Map projections
•Datum transformations
•Coordinate conversions
•CRS (Coordinate Reference System) definitions
In simple words:
Proj4 is the mathematical engine that converts coordinates from one CRS to another.
It is the core behind many GIS and geospatial tools.
Proj4 string is a compact text-based formula that defines a coordinate system or map projection for PROJ, GIS tools, and geospatial workflows.
What is Datum?
A datum is the mathematical model of Earth used by a CRS.
A datum defines:
•The origin point
•The ellipsoid dimensions (size, shape of Earth)
•Orientation in 3D space
There are two main types:
✔ Geodetic Datum (for lat/lon)
Defines the shape of Earth:
•WGS84 (global)
•NAD83 (North America)
•Everest 1830 (India)
•GDA2020 (Australia)
✔ Vertical Datum (for heights)
Defines sea level:
•Mean Sea Level (MSL)
•EGM96
If two datums differ, coordinates differ even if projection is same.
Example:
•WGS84 vs Indian 1954 coordinates may differ by 100–300 m at same physical point.
What is Projection?
A map projection converts Earth’s curved surface (lat/lon) into a flat grid (X,Y).
All projections introduce distortion in:
•Area
•Shape
•Distance
•Direction
Common projections:
•UTM (Universal Transverse Mercator) → very common for engineering & seismic
•Mercator → web maps
•Lambert Conformal Conic → aviation weather
•Albers Equal Area → climate maps
Projection = Mathematical method to flatten the Earth.
How They Relate?
Think of it as a hierarchy:
DATUM → defines Earth's shape
↓
CRS → uses the datum, and may use a projection
↓
PROJECTION → converts lat/lon into X,Y (meters)
Example:
WGS84 (datum)
↓
EPSG:4326 (CRS, geographic, lat/lon)
↓
EPSG:32643 (CRS, projected using UTM zone 43N)
•CRS → the complete definition of how coordinates are expressed.
•Datum → the shape and size of Earth used for coordinates.
•Projection → the math that converts curved Earth (lat/lon) into flat maps (meters).
Input DataItem - connect/reference to Input DataItem usually Read seismic traces/Read SEG-Y traces modules Output DataItem. In case the user wants to use the work mode as trace headers then connect/reference to Output trace headers of the corresponding module.
Work mode { Trace headers, ASCII file } - select the type of input to be considered for. By default, Trace headers.
Work mode - Trace headers - TRUE (Checked) then connect/reference to Output trace headers of the corresponding module.
Input trace headers - connect/reference to Output trace headers of the corresponding module.
Work mode - ASCII file - TRUE (Checked) then provide the input ASCII file with all the information
ASCII file name - provide the ASCII file name
Start row - specify the starting row of the input ASCII file to extract the correct information
X column - specify X-coordinates column range
Y column - specify Y-coordinates column range
Z column - specify Elevation/Z column range
Decimals - if there are any decimal values of Z column, mention the decimal places.
Input CRS - this source/input data CRS coordinates.
Format { EPSG, Proj4 } - By default, EPSG. This is the WGS84 coordinate system
Format - EPSG -
EPSG code - provide the EPSG code. By default, 4326.
EPSG name - it displays the name for code 4326 as WGS84 for easy understanding.
Format - Proj4 - Proj4 is the mathematical engine that converts coordinates from one CRS to another.
Proj4 string - provide Proj4 string name. For example "+proj=longlat +datum=WGS84 +no_defs"
Output CRS - this is the target/output CRS that the user wants the final coordinates
Format { EPSG, Proj4 }
Format - EPSG -
EPSG code - specify the target/output EPSG code. For example, EPSG 23947 (Indian 1954 UTM Zone 47N)
EPSG name - it displays the name for code 23947 as Indian 1954 datum UTM Zone 47N for easy understanding.
Format - Proj4 - Proj4 is the mathematical engine that converts coordinates from one CRS to another.
Proj4 string - provide Proj4 string name. For example "+proj=longlat +datum=WGS84 +no_defs"
Convert elevation - this allows the user to convert the elevations also along with the coordinates. By default, FALSE (Unchecked).
Swap XY - this check box allows the user to swap the coordinates. By default, FALSE (Unchecked).
Auto-connection - By default, TRUE(Checked).It will automatically connects to the next module. To avoid auto-connect, the user should uncheck this option.
Number of threads - One less than total no of nodes/threads to execute a job in multi-thread mode. Limit number of threads on main machine.
Skip - By default, FALSE(Unchecked). This option helps to bypass the module from the workflow.
Output DataItem - generates coordinate converted output data items. Output trace headers will have converted coordinates.
There is no information available for this module so the user can ignore it.
In this example, we convert one CRS to another CRS. There won't be any Vista items available for this module. To QC, the user can use "QC trace geometry" module to verify the conversion.
In the input CRS, we've GDA94 datum and we would like to convert to GDA2020. So, we've provided the Input and Output EPSG codes and executed the module. It will convert all the x,y coordinates from GDA94 to GDA2020.
There are no action items available for this module.
YouTube video lesson, click here to open [VIDEO IN PROCESS...]
Yilmaz. O., 1987, Seismic data processing: Society of Exploration Geophysicist
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