Convert field of view to zoom and back again

This is a snippet from the series described in Playing around with maps and streetview, and shows the calculation to convert between fov and zoom

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1 Objective
2 The result

Objective

The Streetmap image API uses field of view to figure out what part of a panorama to show, whereas streetview in Maps uses zoom levels. This is further complicated because 

  • The max streetview image API size is 640 x 640
  • The zoom levels for streetview are documented as 0-4, but in fact it can go as high as 5. However, in practise streetview stops zooming after about 3 with many panoramas
  • The conversion table given doesn’t appear to give a very accurate result.

Zoom levels are not whole numbers in streetview of course, so you need a conversion formulae. This  can derived from this table 

var fov = 180/ Math.pow (2 , zoom);

and 

var zoom = 1-Math.log(Math.tan(fov*Math.PI/360))/Math.log(2);

However, I find these formulae give a slight better (not quite perfect, but closer) representation

var fov = Math.atan(Math.pow(2,1-zoom))*360/Math.PI;

 

which give these results for zooms 0-4

127
90
53
28
14

The result

Using this formula, to calculate the fov from the zoom generates a pretty accurate set of parameters to pass over to the streetview image api as you can see from the examples below using various zoom levels with each of the APIs. A generated Streetview image API looks like this

The Streetview image API Maps Streetview selection
For more like this, see Ephemeral Exchange