Ya you would think a constant heading would mean a straight line (likely due to seeing 2D maps of the Earth) but on a globe it does not (the equator and meridians being the only exceptions.) Walking along a line of latitude other than the equator isn’t even a straight line. Copy/Paste this into Sandcastle to see the difference between a straight line (geodesic/great circle) and a Rhumbline. Both lines have the same start and end point.
var czml = [
{
id: "document",
name: "CZML Geometries: Polyline",
version: "1.0",
},
{
id: "redLine",
name: "Red line clamped to terain",
polyline: {
positions: {
cartographicDegrees: [-75, 35, 0, -177, 66, 0],
},
material: {
solidColor: {
color: {
rgba: [255, 0, 0, 255],
},
},
},
width: 5,
arcType: "RHUMB",
clampToGround: true,
},
},
{
id: "orangeLine",
name:
"Orange line with black outline at height and following the surface",
polyline: {
positions: {
cartographicDegrees: [-75, 35, 0, -177, 66, 0],
},
material: {
polylineOutline: {
color: {
rgba: [255, 165, 0, 255],
},
outlineColor: {
rgba: [0, 0, 0, 255],
},
outlineWidth: 2,
},
},
width: 5,
},
},
];
var viewer = new Cesium.Viewer("cesiumContainer");
var dataSourcePromise = Cesium.CzmlDataSource.load(czml);
viewer.dataSources.add(dataSourcePromise);
viewer.zoomTo(dataSourcePromise);
There should exist a concept of ‘Great Circle/Geodesic’ heading (I’d say define it as the heading where it crosses the equator going north.) Rhumbline heading means the angle you cross each meridian is the same, but that’s curved along the globe (though straight on those 2D Mercator maps.) Oh ya, lines that are straight on globes are curved on 2D maps.
Great circles could be parallel to eachother (but still intersect) or perpendicular to eachother (90deg apart at intersections.) Equator and Meridians are perpendicular to eachother. Meridians are all parallel to eachother.