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I have a large network of routes that I've created based on an NHD plus stream layer and a large number of points that are located throughout this network. I've got the routes segregated by stream as perfect as I can imagine getting them and the points layer is snapped to the routes. I've QAQC'd and the routes match the streams without duplicates..etc. Everything looks good.

My goal is to determine how far upstream each point is on it's respective stream route. Ultimately this will lead me to stream distances from several fixed points in the watershed to every sample point.

I've been using the Locate Features Along Routes Tool in ArcGIS 10 to do this. At first it seemed like it worked perfectly. But when I looked at the M values for each point I realized that for some routes, measurement started on the downstream end and progressed upstream, giving me what I needed, while on others, measurement started at the upstream end and worked downstream, effectively giving me the opposite of what I need.

I think the definition of the "start" and "end" may be inconsistent among routes (I've checked on this via the Construct Points tool and am pretty sure this is the issue)

Is there a way that I can ensure distance is being calculated in a certain direction on each route?

Any input would be appreciated,

Thanks for your time,

Vin

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If you're scripting, could you put in a work around, where you table the measurements upstream or downstream and make them absolute and just total the absolute values along a given water way. I'm just thinking off the top of my head. Its a bit crude. May work though. –  HowdyDoody Feb 21 '13 at 3:15
    
My best quess from my PostGIS experience is that you dont use right M-values. PostGIS way would be using linemerge to get one linestring to whole route , then using linear referencing etc to figure out length from start to closest_point_on_line. –  simplexio Feb 21 '13 at 9:17
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