Impedance cutoff for demand points and impedance cutoff for Problem Type

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05-13-2013 05:11 AM
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I have 2 types of points, for one group impedance cutoff is 15 minutes, for other 20 minutes. So I did attribute in demand points layer, where it's determined. When I loaded demand points, I choose this attribute  in Property like Cutoff_time (Cutoff_[Impedance]).

ESRI writes in help: "Demand points can override the distance cutoff for the location-allocation problem type. This is useful if some demand points have different needs or behavior. For instance, when prepositioning ambulances, it may be acceptable to reach everyone within four minutes, except for areas with a high-density of elderly people, such as senior centers, which require a faster response time of two minutes."

Now my question. In settings of Problem type e.g Minimize Impedance should I define Impedance_cutoff or not, if I determined cutoff for demand points.
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You do not need to specify an impedance cutoff on the analysis layer if your demand points already have that property specified. You may need to set it if there are some demand points that do not have that cutoff defined and you want an overall cutoff for that analysis. Make sure you are solving on the same impedance attribute that you loaded the cutoff for.

Jay Sandhu
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Thanks for explain, this helped me very much . I loaded demand point and  I determined Cutoff Impedance like attribute with value 15 or 20. I think it should be correct.

Could you me explain why, when I do location-allocation analysis with determined demand points (15 or 20 minutes)  and one time I put in Advanced Settings Tab in  location-allocation analysis window impedance_cutoff 20 and second time I don't fill this field, it will be empty, both analysis are different?
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When you do not put in the 20 minute cutoff and solve, are all the demand points set to provide their own 20 minute cutoff?
Or is it a combination of 15 and 20 minutes? Generally there should be no difference in the solution. If there is, how different is it? Are all the 15 and 20 and 20 minute cutoff being honored? If there is any difference I can only attribute it to the travel direction. For example the travel time from the facilities to demand may not be same as demand to facilities due to one-way or other road restrictions. The demand cutoff of 15 to 20 means traveling from demand to facility should be that much. Where as overall impedance cutoff applies from the facilities to demand.
So is the solution wrong between the two scenarios? Remember Location-Allocation is a heuristic and their can be many solutions that have the same overall objective values (we minimize sum of weighted cost).

Jay Sandhu
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My analysis is a combination of 15 and 20 minutes. 15 for cities and 20 for rural areas.   
That's why I used additional attribut in settings of demand points. And I need a anlysis which respects this situation. The travel direction was this same in both situation and both analysis are different. But this scenario with demand points (with attribute 15 or 20) and impedance cutoff in settings of layer I did only for experiment. If I good understood, in my case I shouldn't fill this field, if I determined impedance cutoff in demand points.

Weighted cost is mean, that  cost e.g travel time to one demand point with weigh 0.5 is lower than this same cost to the demand point with weigh 0.7?
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If you have set all the demand points with the cutoff you do not to set the impedance cutoff on the layer property.

By the direction of travel I mean that the distance from A to B may not be same as B to A due to one way and or turn and other restrictions. So when you set an overall cutoff of 20 minutes it is computing distances from all the facilities to all the demand points that can be reached from the facilities in 20 minutes. When you set a 20 minute cutoff on the demand points you are saying compute distances from demand to facilities that can be reached in 20 minutes. It may not be same. Anyway, it may not be significant issue but may change the solution slightly.

As far as weighted cost, the goal is that when you add up all the weighted costs (demand weight multiplied by the travel time) from the chosen facilities to the allocated demand it will be the minimum. Yes a 0.5 weight will be less than .7 for same distance but what we are doing is locating facilities to allocate the weighted cost so that overall sum is minimum. If the demand is all 1 then we are minimizing overall travel cost.

Jay Sandhu
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Thank you very much 🙂

You don't even know how much you helped me.
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