I'm getting some strange results from a closest facility problem (~40 facilities, ~1100 incidents) in Network analyst. In addition to a Total_Length, which is my impedance measure, I am calculating lengths for each route within several mutually exclusive and exhaustive categories. In other words, when summed for any edge in my network dataset, they add to Total_Length for that edge. Let's call them LengthA, LengthB, LengthC etc. I do this by setting them as Accumulation Attributes.

The resulting Routes look fine, except that the Total_Length of each is not the same as the sum of the lengths by category. The difference is sometimes too large to be because e.g. I am assigning locations to the nearest network location within 5000 meters.

Here's the part that makes me think someone out there will know the answer:

All datasets I am using are in the same feature dataset in a sinusoidal projection. The percent difference between Total_Length and Sum(LengthA, LengthB, LengthC...) is highly correlated (0.85) with the percent difference between the analogous straight line distances calculated using latitude/longitude in the Haversine formula and using projected (sinusoidal) coordinates.

I can't think of any reason why the Plate Carree ("Geographic") or any projection other than the original sinusoidal would be used for one set of calculations but not the other.

I'm running 9.3 SP1 with an ArcInfo license.

Any ideas would be greatly appreciated.

The resulting Routes look fine, except that the Total_Length of each is not the same as the sum of the lengths by category. The difference is sometimes too large to be because e.g. I am assigning locations to the nearest network location within 5000 meters.

Here's the part that makes me think someone out there will know the answer:

All datasets I am using are in the same feature dataset in a sinusoidal projection. The percent difference between Total_Length and Sum(LengthA, LengthB, LengthC...) is highly correlated (0.85) with the percent difference between the analogous straight line distances calculated using latitude/longitude in the Haversine formula and using projected (sinusoidal) coordinates.

I can't think of any reason why the Plate Carree ("Geographic") or any projection other than the original sinusoidal would be used for one set of calculations but not the other.

I'm running 9.3 SP1 with an ArcInfo license.

Any ideas would be greatly appreciated.

First are you solving on Length? OR are you solving on say, travel time and accumulating Length to get the Total_Length field?

Second, how was the Length attribute added/defined in the network dataset?

Third, how are the length by category attributes set up? Are they based on the same "Length" attribute of that edge? Or were they set up by some other formula?

Network Analyst itself is not doing any projections to figure out the lengths of the edges. And it will accumulate what ever fields you ask. There is no error in simply summing up attributes to report out the accumulation of the various attributes. Any differences have to exist in the input data.

If you use the network identify tool (second icon from right on the NA toolbar), it will list out all the attribute values for that edge. Use that to see if the length and the length by category are the same. Most likely they are not same and that is source of the differences.

Regards,

Jay Sandhu