In Summarize Within tool, we need to choose "Geodesic" or "Planar" calculateion.

[MitsuyoTAKAGI]

I put this idea on behalf of the end user.

I tried to get a sum of the area by Summarize Within tool like the attached picture, but the value is not what I expected because the Summarize Within tool uses “!shape.geodesicArea!”,  while Shape_Area field is calculated by “geometry.getArea”. (Esri support told me this in the case 02269220)

My data is PCS (Projected coordinate system). Generally, the distance or area of the PCS data is calculated by "PLANAR", while these of GCS data is calculated by "GEODESIC".

To summarize the area or length for PCS data, the parameter for "PLANAR" is necessary !

[PaulTudman1]

Thank you for posting this Mitsuyo - it was driving me crazy trying to work out why the area calculations were incorrect when using the Summarize Within tool. 

I cant believe more people don't notice this is happening and post about it - most people seem just run with the results they get these days and don't actually cross check them to see if they make sense. 

This is a big oversight on ESRI's behalf and should be fixed ASAP.

[RebeccaAdamson]

Agreed!  All our calculations are planar, but this function wants to use geodesic... I've got to do them manually just so that I know I have consistency, I need a model or some batch processing or something, I've hundreds of polygons to calculate.  I noticed that if the CRS for a layer is not projected, when Calculating Geometry, the option isn't there to project on the fly and take an area calculation.   This is logical, the parameters are already specified in a layer with a projection system, whereas requesting an area in a particular projection would require specifying the transformation to use.  However, if I can add a WGS84 layer onto my projected map and get an on-the-fly projection to display it, it's reasonable that I might want to calculate an area relative to other layers also projected, and the map has the projection system and transformation specified inherently.  So why can't I do this or specify it manually in either case. Gah. 

Sorry for the rant, it's taken a lot of time to explore the issue and work out what I have realised I cannot now do.  Having told the stakeholder that I'd have it done by the end of the day because this is what it's designed to do 😞