Sunday, August 19, 2018

Transforming Nigeria Minna Coordinates (epsg: 4263) to WGS 84 (epsg: 4326) using QGIS

Introduction:

Different countries have different local coordinate geographic reference systems (CRS), Nigeria is not an exception. One of the most developed and used local geographic CRS in Nigeria for general mapping is the "Minna  (epsg: 4263)".

EPSG - "European Petroleum Survey Group" is the organization in charge of maintaining different datum, coordinate reference systems and coordinate transformations around the world which may be global, regional, national or local in application. It is also worthy to know that the organization is also called: "International Association of Oil and Gas Producers" (IOGP).

A search on EPSG.io for "Minna" reveals both geodetic and projected CRS. Our focus in this article is on the newest Minna geodetic (geographic) datum with EPSG code 4263, that has it measuring units in degrees as seen below.


Note that there is a deprecated version of the Minna datum with EPSG code as: 62636405

"Minna" geodetic datum was first derived from the "Nigerian National Coordinates System" which is no longer in use. "Minna" geodetic datum is suitable for use in Nigeria - onshore and offshore. "Minna" references the Clarke 1880 (RGS) ellipsoid and the Greenwich prime meridian. Minna origin is Fundamental point: Minna base station L40. Latitude: 9°38'08.87"N, longitude: 6°30'58.76"E (of Greenwich). Minna is a geodetic datum for Topographic mapping.

It has the following Extent: 2.66°E, 1.92°E, 14.65°N, 13.90°N (upper long., lower long., upper lat., lower lat.).



WGS84 on the other hand stands for World Geodetic System 1984 and comprises of four different things:

1~ An ellipsoid
2~ A horizontal datum
3~ A vertical datum
4~ A coordinate system

The WGS84 is the datum used by the Global Positioning System (GPS). The datum is defined and maintained by the United States National Geospatial-Intelligence Agency (NGA). Coordinates computed from GPS receivers are likely to be provided in terms of the WGS84 datum and the heights in terms of the WGS84 ellipsoid.

Today, WGS84 is the standard for GPS, Google Maps, Bing Satellite, and many other GIS and mapping platforms. It uses the Earth’s center mass as the coordinate origin. Geodesists have computed and found that the difference of approximately 150 meters existed between Minna datum and WGS84 datum. This means, when you use a set of coordinates that has it datum in Minna on a map platform based on WGS84, you will have a displacement error of ~150m.

The map below shows points in Minna datum plotted on a WGS84 platform. The displacement is seen on below...


As you can see from above, the project CRS is in WGS84. The green points are in the correct CRS and in there correct locations. While the orange points are actually in Minna datum but mistaken to be in WGS84, hence they are displaced by an average distance of 87m from their true/correct positions. This is way too much displacement to be ignored by any mapping project supervisor.

It is usually a common problem faced by mapping practitioners in the country.

Transforming coordinates in Minna (epsg: 4263) to WGS 84 (epsg: 4326):


Step 1: Save your Minna coordinates in a CSV file



Step 2: Go to Layer >> Add Layer >> Add Delimited Text Layer...



Step 3: Click on “File name” button and browse to the location of your CSV data to load it. Make sure, “x field” and “y field” are selected correctly as seen below.



Still on the same window, click on the “Geometry CRS” button then filter it using code “4263”, and select it and click on “Ok” button as seen below.


Now, click on the “Add” button on the previous window, then click on “Close” button. If everything went well, you should see the points added onto the map area.



Step 4: Right-click on the Layer name, and select "Save As".




Step 5:

Under "Format", choose "Comma Separated Values (CSV)" as the format and specify the "file name" as well as its location. The crucial step here is specifying the CRS WGS84 (click the button and search for code 4326). Lastly, before you click on “OK” button set “Geometry” to “AS_XY”. See screen shot below…



Step 6: Now you should have a new file in your working folder (with whatever name you choose when saving it from above) that will contain the converted coordinates.


Note: If you wish to, you can convert the output CSV file to Excel and also edit the column names from "X and Y" to something else as per you need.


NOTE: The steps outlined above also work for the conversion from projected Minna/UTM zone 31N (epsg:26331), Minna/UTM zone 32N (epsg:26332), Minna/Nigeria East Belt (epsg:26393), Minna/Nigeria Mid Belt (epsg:26392) and Minna/Nigeria West Belt (epsg:26391) to WGS84 (epsg:4326).


References:

1- Origins of Selected Geodetic Datums
2- Surveyors hold workshop on transformation parameters   
3- The Nigerian National Datum: The Call for Further Realization  
4- Geodetic Datum used in Nigeria     
5- What is WGS84?  

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