The position of any point on, above or below the earth’s surface can be uniquely defined by assigning it coordinates along three axes…
X (latitude or northing) — Y (longitude or easting) — Z (elevation)
Most maps are 2D orthographic plan maps in that the X and the Y coordinates only are projected onto a flat surface, usually a paper sheet or screen. The viewer is looking vertically down along the Z axis so there is no indication of elevation.
Cartographers get around this by using various techniques to help visualise the terrain in 2D. Common examples include contour lines, spot heights, hillshading and hypsometric colour.
Many people find maps easier to relate to if they are displayed in three dimensions rather than two. A 3D map has all three coordinates (X, Y, Z) projected onto a flat surface. To create a 3D map we drape a digital 2D orthophoto or texture map texture over a digital terrain model, much as you would drape a blanket over a bed.
2D orthographic plan maps have parallel projection lines. The viewpoint is at infinity. However 3D maps usually have a fixed viewpoint or camera position. The view is a perspective with converging projection lines.
A 2D orthographic plan view (left) and a conventional 3D perspective view (right)
- there is usually large variation in direction and scale with distant features appearing much smaller.
- there is a focus on the centre of the view and peripheral areas appear distorted.
- significant areas in the view tend to be occluded (hidden from view).
Parallel Projected 3D Maps
3D perspective maps have issues because they have convergent projection lines and fixed viewpoints. So why not make 3D maps with parallel projection lines and a viewpoint at infinity?
The Plan Oblique projection does just that. So does the Orthographic Oblique projection which is a derivative of the Plan Oblique. We generally prefer the Orthographic Oblique projection because the view appears more natural. Both have parallel projection lines, a viewpoint at infinity, and both are largely free of the issues associated with perspective 3D maps
Plan Oblique projection (left) and Orthographic Oblique projection (right)
The Road to Milford Sound – an orthographic oblique 3D map…
What we need to know from you…
A 3d perspective view (left), a plan oblique projected view (centre), an orthographic oblique projected view (right). The elevation angle in all 3 examples is 35°.