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How is the map scale for the scale bar in the ArcGIS Online map viewer calculated?
ArcGIS Online uses the location of the scale bar to calculate the map scale in the map viewer.
In the ArcGIS Online map viewer, when the scale bar is anchored at the bottom left corner, the bottom left corner is the location at which the scale bar is measured. The scale bar becomes shorter as users pan a map towards the Equator at the same zoom level. When viewing a map in the Northern hemisphere and panning South, the scale bar becomes shorter. When panning the map South in the Southern hemisphere below the Equator, the scale bar becomes longer.
By default, ArcGIS Online uses the WGS 1984 Web Mercator (Auxiliary Sphere) coordinate system that preserves direction and the shape of data, but distorts distance and area. However, the ArcGIS Online map viewer uses the Geodesic method that accounts for the curvature of the earth in regards to projecting the basemap layer, and the scale bar considers the projection distortions and dynamically adjusts the scale bar.