Measurements are inaccurate in ArcMap 10.4.1

[PeterPeregrine]

When using the measurement tool in ArcMap I am getting measurements that are far longer (by about 25%) than known distances on all images in my map.  Measurement is set to Planar and I cannot change it.  Coordinate system is 1984 Web Mercator auxiliary sphere.  Any ideas how to fix this?

[DanPatterson_Retired]

web Mercator should never be used for measurements of distance and area.

Pick a different projection.

There are lots of links on this...

here is one from the Near tool that stipulates that geodesic calculations need to be used when you have inappropriate projections

It is strongly suggested to use the Geodesic method with data stored in a coordinate system that is not appropriate for distance measurements (for example, Web Mercator or any geographic coordinate system) and any analysis that spans a large geographic area.

[PeterPeregrine]

What would be a good projection to use?

[DanPatterson_Retired]

depends where you are and how far the distance is.

geodesic calculations are appropriate for anywhere

If you need a projection and your width is 6ish degrees wide, the ever favorite UTM. then there are dozens of others which cover larger extents.

So, the whole field of coordinate systems and projections are open to you, just pick one that is commonly used in your area and is appropriate for the distances you are measuring and you understand the limitations of your choices.

[PeterPeregrine]

I'm an archaeologist and I really don't know much about cartography or GIS, so I could really use some advice on an appropriate coordinate system and projection.  The distances I am measuring are short--on the order of 10-20m, and I am not able to use geodesic with the Web Mercator system (all but planar are greyed out).  Any help from you or others would be most appreciated.  Thanks.

[DanPatterson_Retired]

what is your map that you are measuring from? a basemap?

10-20 cm is really short, so find a UTM projection for your longitude

[PeterPeregrine]

It doesn't seem to matter if I measure from the basemap or one of the raster images I have layered and georeferenced on it.  The basemap is "World Imagery" from the ESRI site.

[BENNYROBERTS]

Peter, I totally agree with Dan. I use ArcMap for mapping archaeological sites and always use the UTM projection for the longtitude of the site. I have "ground truthed" some georeferenced raster measurements and they have always been spot on.