The distance measured by the circle and line segment in OpenLayers is not the same. about openlayers HOT 6 CLOSED

The radius of a circle geometry, despite being in map units (meters for EPSG:3857), is not actually a measurement for the distance on the ground.

You need to use methods from ol/sphere to actually get the great sphere distance given the radius of the Earth.

For instance, you could use ol/sphere -> getLength() against a geometry created from the extent of the Circle geometry in question, and divide that by 8.

When doing that, I get the same distance as measuring with the line segment per the openlayers example.

from openlayers.

Thank you very much for your answer.
However, I still don’t quite understand why we divide by 8.
What I can think of is based on the distance measured by the line segment.
I will give it a try.
Thank you.

from openlayers.

However, I still don’t quite understand why we divide by 8.

For the particular trick I did, getLength() adds the length of all four sides of the extent together, but we only want the length of half (/ 2) of one of the sides (/ 4).

An alternative using getDistance requires geographic coordinates, so not exactly any simpler. You could also use getLength using the circle center and a point on the radius.

from openlayers.

I truly appreciate your help.
Unfortunately, my English is not good enough, and I'm a beginner, sigh!
I constructed a line segment with the center of the circle and the last coordinate of the circle as the two ends,
and then measured their length.
The problem is solved, but I found a strange phenomenon, which should be due my misunderstanding.
Here is the code:

const line = new LineString([
	[-11000706.48267012, 4599925.709030858],
	[-10998658.283823326, 4599947.8227707315]
]);
console.log('sphere.getLength()--->', getLength(line), ',line.length()--->', line.getLength());
// ===> sphere.getLength()---> 1609.098756749408 ,line.length()--->'2048.3182207611635

from openlayers.

Like the circle's getRadius(), line's getLength() is not an actual measurement on the ground.

This is due to the default Web Mercator projection -- as you get closer to the poles, the apparent size of things gets larger. (This is common in many other projections, which is outside of the scope of a simple GitHub comment)

The getLength() function in sphere actually accounts for this which is why it returns a different (and accurate) value.

A good starting point that might help you understand might be here: https://en.wikipedia.org/wiki/Tissot%27s_indicatrix?wprov=sfla1

from openlayers.

Thank you. I have check it out.

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