I have test it a bit and seems to work correct. Inspired from this
var res = getResolutionFromScale(200000);
alert(res);
function getResolutionFromScale(scale){
var units = map.getView().getProjection().getUnits();
var dpi = 25.4 / 0.28;
var mpu = ol.proj.METERS_PER_UNIT[units];
var resolution = scale/(mpu * 39.37 * dpi);
return resolution;
}
First, a note about map scales.
A map scale represented as a unitless factor (e.g. 1:50000) means that a map unit represents X map units (e.g. a meter in the map = 50 km in reality).
This kind of scale is simply impossible to achieve in web maps, as you cannot be sure of the physical screen size. Think about a laptop connected to a projector screen, having the same image in the small laptop screen and the huge projector wall at the same time.
Now, the usual assumption is that computer screens have a resolution of 72 or 96 pixels per inch, and then one can calculate the size of a CSS pixel as 0.26mm to 0.29mm.
What Leaflet (and any other web map engine) does is have a scale factor between (CSS) pixels and map units in the display CRS. In most web maps, this means a relationship between pixels (in CSS) and EPSG:3857 CRS map units (represented as meters, only makes sense in the equator because projections are complicated, which will add to the problem).
The magic of the "usual" tiling scheme is that the circumference of the earth at the equator is represented in 256 pixels at zoom 0. This means a scale factor of approximately 40075km/256px ~= 156543m/px.
This is meters in the display CRS. As the common CRS (EPSG:3857) implies a cylindrical projection, which is conformal but not equidistant, the pixel-to-meter scale only has meaning at the equator (this is why the scale bar in web maps shifts as you pan the map north or south, by a factor depending on the cosine of the latitude).
So, a Leaflet map does have a map units to pixel scale per map unit (156543units/px). By default, the scale halves at every zoom level.
So now the question becomes: «How can I manually specify a map units-to-pixels scale to Leaflet?»
There are several approaches.
One is to create a custom CRS, specifying scale and zoom functions which fits your needs. This requires a bit of knowledge about extending Leaflet, in order to not make a mess of the code. You'll also need to generate raster data (tiles) according to this scale/zoom relationship in order for the map to appear crisp.
Another approach is to use the fractional (i.e. non-integer) zoom capabilities in Leaflet 1.0.0. Given a map units-to-pixels scale factor, calculate the zoom level (using a base-2 logarithm), set the zoomSnap option of the map, and set the zoom level to the desired value. Note that raster tiles might appear blurry (as one raster pixel will not map to one screen pixel).
A third approach is using Proj4Leaflet. This is basically the user-friendlier way of defining a custom CRS, without having to delve in the particulars of the L.CRS implementation manually. Note that Proj4Leaflet allows you to specify a list of pixels-per-projection-unit to be used as zoom levels, making your life a bit easier. Again, you might need to generate your own raster data if you want it to look crisp.
Best Answer
The easy thing you can do is to display a scale line, as demonstrated in this example.
Computing a correct fractional scale is quite complicated, because you would have to take into account the resolution of your screen, which of course differs from one device to another. This control did exist for OpenLayers 2.