Drive Direction Distance does not match Measure Tool

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11-06-2017 11:53 AM
MichaelThompson
New Contributor III

Please refer to screenshot attached.

I calculated a route along the entire length of a street segment feature.  The Cost value is consistent with the data, but the Distance value is way off. The Drive Directions says the path distance is 1098 feet, but the Measure Tool says 815 feet - a deviation of about 35%. 

The Distance attribute of the ND is using Shape_Length which is in decimal degrees, but I have Feet specified as the Distance Display Units in the ND Directions Agent. It seems that the conversion from Decimal Degrees to Feet that the Directions Agent is using is not consistent with the conversion options in the Measure Tool, unless I've made a mistake somewhere. Any ideas? Thanks in advance!

Michael

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DanPatterson_Retired
MVP Esteemed Contributor

I was close in my guess then...  My thought on that query related to the observation that your line was almost parallel to the lines of latitude and the spacing of lines of latitude decrease by cos(lat)   I took your 'correct' distance and found the latitude that produced your 'incorrect distance', coming up with about 39 degrees hence, it wasn't a geodesic conversion nor a straight unit conversion so cos(45) * 1098 ft = 803 ft .... off of yours by some 12 feet, but geodesics use meters and I didn't think that the US feet versus international feet would matter (maybe it does, but it should be consistent.

MichaelThompson
New Contributor III

Thanks for your guidance!

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MichaelThompson
New Contributor III

ESRI is now saying this works as designed because Shape_length does not include any spatial reference, so they have to use some default projection. This makes sense, and the moral of the story is to always use Shape instead of Shape_Length for your distance attributes whenever you are needing the Drive Directions agent to convert distances between geodetic and planar values.

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