ArcGIS Online vs. ArcMap_Difference in measurement

[KimberlyTejeda-Lankford]

I started lines from my feature class layer my map in Arcmap (Desktop), I then went on ArcGIS online and found that the length of the line that I was putting in ArcMap doesn’t match when I measure again in ArcGIS online. For example, a line that I put as 900’(planar) in Arcmap, shows up as 753’ online (I believe is in WG S84). How do I change my system in ArcMap, so that when I specify a length of 900’ in shows as 900’ online. Also if I change the measurement to Great Elliptic on ArcMap is shows the 754’. mY contractor measured the distance between my clean outs and the measurements line up with the length shown on ArcGIS online.

[DanPatterson_Retired]

ArcGIS Online uses a web mercator and it isn't suitable for distance or area measurements.

This goes way back

FAQ: Why are my map, distance and area measurements wrong when using WGS 1984 Web Mercator? 

Measuring distances and areas when your map uses the Mercator projection 

So make sure you aren't using that coordinate system when making those measurements unless you take steps to account for this

[MichaelMatson]

Hi, I'm having a similar problem as the original poster. 

While I understand why map projection differences can cause errors, I am trying to measure distances at small scales (300 meters and less) and I am getting massive differences in what ArcMap desktop returns and what I measure on ArcMap Online.  I don't understand how differences in measurements (planar, geodesic, etc...) or projections could cause differences in returned values from either platform at these scales. 

Some examples include a measurement of 434ft in ArcMap desktop (UTM 10N) but 348ft in ArcMap online (20% difference), and an area of 26 acres on desktop and 17 acres online (35% difference).  As the original poster also mentioned, the online measurements are probably the correct measurements, so how do I reconfigure ArcMap desktop to give the actual values?

[NathanBruce1]

I have not tried this yet, but makes me think that its a potential solution, Basemap is defined as a projection other than web mercator. 

How to change the projection in ArcGIS Online in one easy step