HELP please!!

[dannyg77]

I am a grad student trying to work on a simple Vensim model - my first one - and I can't seem to make the lookup feature work for me. Caution - this is the kind of question that is so basic you might not think it is worth your time to answer. My apologies for placing such an easy question on this forum.

I have a simple model using only one level which represents water in a lake. I had a constant to represent the surface area of the lake, which of course, is moronically simplified because the surface area of the lake depends upon the volume of the lake. I have a small data set (nine points) that describes the the surface area of the lake with respect to the total volume. From there I get confused. So the questions: do I change surface are from a constant to a lookup? How do I get the program to realize that the surface area is supposed to be derived from the lake volume? Is the arrow linking these two items supposed to go from the level to the lookup or vise versa?

dannyg77

ps e-mail is dannyg77@yahoo.com if you want to send me a nasty gram telling me to bugger off your forum.

[LAUJJL]

It would help anybody that wants to help you to know the objective of your model first. What are you trying to do? The fact by example that you know the lookup relation between the amount of water in the lake and the surface, questions the necessity to represent the lake as a level. Are there flow in and out of the lake?
The model could eventually calculate the surface of the lake depending from the time, then calculate the quantity of water by the lookup, then by difference the net flow in and out of the lake, which reverses the usual dependence of the level from the net rate.
Joining a very simple model of yours could help to understand the problem.
Regards.
J.J. Laublé

[Wilson]

have a look at the attached model, this may help you.

[dannyg77]

Thanks! Turns out my main bit of confusion was that I was using a Stella textbook and working with Vensim. Most of the time, I can deal with the differences. But the way that Stella and Vensim handle lookup tables is a bit different.

Thanks for the lake model. I really appreciate it.

I would upload the lake model that I whipped up but I don't know how... let's see... I suspect that I don't have the priv to do that. Well, cheers anyway! I expect to become a regular here.

Merry Christmas (oops, politically incorrect)
Dan

[Edited on 12-12-2004 by dannyg77]

[LAUJJL]

When you have loaded the model into one of your directory, it has an *.htm name. You must copy the file or rename it to lake.mdl.
When you open that file, it will show you the text format.
When in text format if you try to look at it as a sketch (view function then as sketch ) if will tell you that there is an error. Erase everything in the text format from (!DOCTYPE to <PRE> that is in < > before the inflow equation.
The text should start after the erasing with inflow =
Random uniform etc..

Go back to the view and to the as sketch. It should work.
There is may be another way to deal with the loading, but I use this one.
Regards.
J.J. Laublé

[dannyg77]

To upload the model, can I just zip up all of the files that are needed? If so, what are all of the files that I need to zip? Not that anyone would be interested, but considering the state of panic I was in and the pathetic cry out for help, I thought I would at least let you all see what I was trying to do.

I will give it a shot hoping that the files needed are in the directory I saved my work.

Dan

PS I have been using the program for three days now. It's fantastic.

[Edited on 12-13-2004 by dannyg77]

[LAUJJL]

I tried to unload your zip file, got it but could not open it with Winzip
There must be some reasons.
The administrators should know why.
Regards.
J.J. Laublé

[Wilson]

I can download and simulate your model without any problems. Looks good for someone only having used Vensim for 3 days.

Also, I haven't encountered any of these issues which require you to download & rename the model. But I will try to look into it further.

[dannyg77]

Oh, one thing I forgot to make perfectly clear. That model is out of a book by Dr. Andrew Ford of WSU. I did the Vensim simulation of the model to learn how to use the software and to learn how Vensim applies uncertainty. The credit for the model goes to Dr. Ford and his students.

Dan