The Influence of the Projection Plane Altitude on the Distortions in Oblique Double Conical Map Projections

In double map projections, the Earth's surface – approximated as a revolution ellipsoid – is first represented on a sphere that is then subsequently represented on the plane of the map projection. In the present study, the ellipsoid is conformally represented on a sphere, which introduces linear and area distortions. Herein we have studied the representation of the sphere on the lateral surface of a cone chosen such that the tangent parallel passes through the middle of the area of interest. Additionally, in this case, linear and area distortions also occur. The total distortions produced by double map projections must include both the distortions arising from representing the ellipsoid on the sphere, as well as those appearing when representing the sphere on the plane. Said distortions depend on the position of the point on the surface of the ellipsoid or the sphere, as well as on the latitude of the parallel which is represented as undistorted. The projection plane altitude also has a influence on the produced distortions, as can be demonstrated by changing the altitude of the projection plane. In the current article, we present this scenario for several road sections in Romania that were represented in local projection plans located at the average altitude of their respective location.