Can't calculate area of Thiessen polygons, raw or dissolved.

[Tamsen_TremainByfield]

Hello,

A few questions:

1) I have X,Y point data from which I've generated Thiessen polygons as one layer. I then clip this layer to another to get my final boundaries and dissolve by an attribute (habitat class). In the attribute table of each, I've added a new field and tried to run "field geometry" to get the total area of each polygon. The "area" selection in the dropdown box is "disabled". Does anyboyd know why I cannot generate the area of each Thiessen polygon?

2) The Thiessen polygon attribute table has lots of "Null" values entered in cells that previously held data. ????

3) In the final dissolved layer there are several wispy lines (incomplete polygons). Too many to delete manully via the Edit tool.

Answers in simple, advanced beginner's GIS language would be most appreciated.
Thank you.

[MelitaKennedy]

My guess for the area calculation is that the coordinate system is a geographic one: latitude/longitude and thus degree-based so it doesn't make sense to calculate "square degrees". Try reprojecting the data to a projected coordinate system that preferably uses an equal area-based projection like Albers or Lambert azimuthal. If the area doesn't have to be that accurate, use the local UTM or state plane zone (both use conformal projections which maintain shapes, not areas).

Melita

[JakeSkinner]

1) I have X,Y point data from which I've generated Thiessen polygons as one layer. I then clip this layer to another to get my final boundaries and dissolve by an attribute (habitat class). In the attribute table of each, I've added a new field and tried to run "field geometry" to get the total area of each polygon. The "area" selection in the dropdown box is "disabled". Does anyboyd know why I cannot generate the area of each Thiessen polygon?

2) The Thiessen polygon attribute table has lots of "Null" values entered in cells that previously held data. ????

3) In the final dissolved layer there are several wispy lines (incomplete polygons). Too many to delete manully via the Edit tool.

1)  Your data is most likely in a Geographic Coordinate System.  Set your data frame to a projected coordinate system by right-clicking on the data frame > Properties > Coordinate System tab.  When calculating the area, choose to the use the projection of the data frame.

2)  Is your output a shapefile or feature class?  If it's a shapefile, run your process again but write your output to a File Geodatabase and see if you still receive the NULL values.

3)  After you calculate the area, you could perform a select by attribute to select all the polygons that have an area less than a certain value, then delete these all at once.

[Tamsen_TremainByfield]

Thank you both for your replies. Very helpful.