Soil acreage calculations issues

[MatthewVance]

I am attempting to derive the acreage of soils that are within the boundaries of a parcel.  I've been clipping the soils to the parcel I'm interested in and using the shape area field for the clipped soils in order to calculate my acreage.  The problem is that when I add there results together I end up with an acreage that does not match the acreage I have for that parcel in the original survey.  Am I going about this the wrong way?

I'm also looking to create a model where you select the parcel you want and I comes up with the acreage for each soil within that parcel.  Any tips for that would be appreciated.

[DarrenWiens2]

Are the values off by a little (rounding, resolution, etc.) or a lot (wrong units)?

[MatthewVance]

It varies.  There seems to be a correlation with the size of the parcel but I haven't determined if that's the case or not.  My data frame is set to display feet so I assume the units in the area field are square feet.  I've just been using the field calculator to divide the area field by 43560 in order to get the acreage.  The parcel I've been testing this with should have 117 acres yet when I add up the total acres of the soils on the parcel I only come up with 88.

[DarrenWiens2]

I assume since it's in feet, you're working with State Plane Coordinate System, Lambert Conformal Conic Projection - which will introduce some area distortion but I doubt it would account for losing 29 acres, unless you are working way outside the standard parallels. Is this your projection, and are you within the standard parallels (e.g. the US)? If not, where is your area of interest and what projection are you using?

Other than that, I'd suggest looking closely at your before and after acreages to be certain the numbers are what they should be (e.g. was the before area calculated in a static field, then geometry changed?).

[MelitaKennedy]

If the original value came from a ground survey, you're unlikely to match it, although that seems quite a bit off! You may be running into a few things: projection used to calculate the areas isn't equal area, or a grid vs ground, or both.

The data frame may be using a state plane or UTM zone. Both are conformal and maintain shapes, not areas. A projected coordinate system is also done relative to the ellipsoid (spheroid) surface, not the elevation/ground and that can also cause distortion.

1) How close are the areas of the entire parcel?
2) Try using an equal area projection like Albers or Lambert azimuthal, modified to put their centers close to the data

Melita

[MatthewVance]

I'm using the Ohio South State Plane System.  My AOI is Lawrence County Ohio.  You may be on to something with the area being calculated in a static field and the geometry changed.  The parcel shapefile was created by a third party before I was hired and our engineer's office (i'm in the auditor's) has been updating it using CAD.  I'm sure there was a lot of "the shape is right, now make it fit" thinking going on.  The amounts I'm off by seem rather extreme though.

The original value did come from a ground survey.  I changed to an equal area projection but there is no change in the total area of the parcel.