I am trying to make a polygon from the aviation LPPO FIR (Santa Maria Oceanic Flight Information Region), that some portion is described as "arc of circle with 100NM radius centred at PST NDB (anti clock-wise)".
DATA (DDMMSS):
- PST NDB : 330407N 0162130W
- Start point of arc : 341504N 0174605W
- Ending point of arc : 3215N 01438W
In QGIS 3.24.3-Tisler:
- Fresh project in EPSG4326
- Plotting DATA coordinates in a point layer (EPSG4326)
- Making a circle with vector_geometry/buffer or python_script – Layer (EPSG4326)
Can obtain a visually close "perfect circle" (adding decorations/grid seems a perfect circle) but with a radius of ~99.455NM in Y axis, and only ~85.865NM in the X axis with measure_line/elipsoidal (in cartesian Y:~99.813NM X:~102.355NM)
the drawn circles also do not reach the points of start/end of arc of circle.
adding a layer with the actual boundary for comparison, the draw circle has the right dimension on N/S but wrong in E/W – the actual shape should be more close to a horizontal ellipse.
Work around
Using online Vicenty calculator, determine 2 point at a distance of 100NM from PST NDB and with 180º and 270º (respectively south and west) and then using "add ellipse from center and 2 points" with the PST NDB and this 2 new points to draw an ellipse that looks closer to the layer with the actual boundary.
Question
What is the issue here, knowing that maintaining the same CRS /EPSG 4326.
Is there any path to do it properly, and without the need to create the 2 additional points for reference?
or using the alternative method below.
Additional Information
Plotting a circle using the https://www.fcc.gov/sites/default/files/circle-plot.html and importing the KML to QGIS, the 100NM radius "circle" measure ~99.767NM in X and ~100.225NM in Y and superimposes all the expected coordinates looking pretty close to the expected.
Best Answer
Your problem stems from the use of EPSG:4326 when generating your arc. While 4326 is fine for locating point data, it should not be used with anything that involves distance or area (including line or polygon data). That's because 4326 uses lat/lon values instead of meters or feet, and the distance (expressed as degrees) between any two lines of longitude shrinks as you move from the equator to the poles. At the poles the distance becomes zero. You can see this by observing a globe. Meanwhile, the distance between any two lines of latitude stays relatively constant (it would be constant if the earth was a perfect sphere, but it bulges a little here and there, causing small variations in the distance between parallels).
You can see this in your results: The Y-axis (latitude) is pretty close to 100nm, but the X-axis (longitude) is way off.
The solution is to convert the point data from 4326 to a projection that uses meters or feet. Here's the approach I took: