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I was wondering what the minimum threshold was for the minimum area of a modeling strata.
I was noticing in the run dialog box, the following message was occurring for small strata:
“No Valid # of simulations cells provided. Model defaulting to 100.’ I only wrote down the strata # for a few of these messages, so after the entire run of all the strata, I found that this was occurring for a strata of 1 acre and for a strata of 24 acres.How is does Path determine what a valid area is? Is it an absolute area or is it dependent on the # of states within a strata?
Optionally:
-Is there a way to manually change this threshold in Path?
-Is there a way to prevent these non-valid strata from being modeled instead of defaulting to an artificial area?ILAP normally “lumps or dumps” small strata in our data rollup process (before uploading the strata into Path) so I haven’t noticed this issue before. But I would like to discuss this issue in a data rollup tutorial I am creating, so that the user will be able to make an informed decision on whether they should use or ignore our lumping tool for their project area.
To better understand the problem you are experiencing, it would be helpful for me to know what version of Path you are running and to get a copy of the database you are using. Would you be able to post it to ESSA’s incomming FTP directory (ftp://ftp.essa.com/incoming/) and let me know when it is uploaded?
It could be that your strata are so small that the number of simulation cells being assigned to them is equal to zero. This would happen if the proportion of the total area made up of that stratum multiplied by the total number of cells being used for the simulation is less than 1. Note that this should no longer be a problem in the upcoming version 4.0 of Path that has it’s own simulation engine, rather than relying on VDDT.
Cheers,
LeonardoThanks for posting your database. In your specific case the issue was caused by the following:
In version 3 of Path which you are currently running, the number of simulation cells to run is calculated as the total area set by the user in ac divided by the resolution set by the user. Your model uses a resolution of 200*200 m or roughly 10 ac. The number of simulation cells is calculated as an integer so anything less than 5 ac in size will have a calculated number of simulation cells of zero and will result in the message you describe.
In Path version 4 the user sets the total area and the number of cells rather than the resolution. The user interface will not allow you to set the number cells to a value less than 1 so you should not encounter this problem. In the current beta release of version 4, when you upgrade your data from the previous version it will calculate the number of simulation cells based on the formula described above and you may end up with scenarios with zero simulation cells. This is something we will fix in the full release of version 4.
For version 4, did you change the database validation rules in the table field properties or did you change something in the Path software? The ILAP “GIP tool” that generates modeling strata may create a strata that’s less than 5 acres. So will strata such as this be added as a record or do the validation rules prevent this from occuring? (In which case the GIP Tool may not work). If you changed the Path software, however, does this mean that these small strata will be created but not run or return results, or???
In version 4 we changed the database schema so the CEAGeneral Table which specified resolution and total area has been changed to the RunControl table which specifies number of simulation cells and total area. I imagine that the “GIP tool” will need to be modified to work with this schema. Note that if you are entering data directly into the database without the UI there is currently no validation rule on the number of simulation cells so you want to be careful about entering a value >=1.
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