Grid data from a shape file

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epelde.ane@gmail.com
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Joined: Thu Jan 01, 1970 00:00

Grid data from a shape file

Postby epelde.ane@gmail.com » Thu Oct 20, 2011 09:25

Hello again :)
I´m wondering what happens when the polygons of a shape don´t take up the whole extension of the grid I´m using. Is it in this case when the default value is used by mohid?
Thanks!

Jauch
Site Admin
Posts: 98
Joined: Thu Jan 01, 1970 00:00

Re: Grid data from a shape file

Postby Jauch » Thu Oct 20, 2011 09:52

Hi Ane :)

If by "default value" you mean the keyword used in many places in the input files, like the properties on the atmospheric input file (precipitation, relative humidity, etc), then, the answear is no.

The "default value" that I described above is used only if the data for the property is not given by a file (grid data, hdf, timeseries, etc).

When you use a file as input to the property, lets say a grid data, each cell must have a value.
If you are saying to mohid to delineate the basin (on the basin geometry file), then it will do the simulation looking only for the cells inside the basin and will ignore all the cells that falls outside it. If you are not delieating the basin, than, if I'm not mistaken, you can use the topography grid data to tell to mohid what cells you want to use or not. I think the value to tell that a cell will not be used is -99. Usually will be the value set by the FILL_VALUE keyword in the grid data file.

When you create your grid data from a shape file, using mohid studio, there is an option to you to set the fill value of the grid data. I think it's set to -99 by default, but I'm not sure. The mohid studio will set all cells that fall off the poligons to this value.

So, depending on what you're doing, you need to pay more attention to what value you will use as fill value when constructing a grid data from a shape file, or you may need to edit manually these values to the ones you think are approprietade.

:)

epelde.ane@gmail.com
MOHID Advanced user
Posts: 172
Joined: Thu Jan 01, 1970 00:00

Re: Grid data from a shape file

Postby epelde.ane@gmail.com » Thu Oct 20, 2011 11:07

Ok! :)
So, just to get sure I didnt misunderstand it:

- the "DEFAULTVALUE" keyword within each property is not taken into account when I input time series for that property. This is just used when you give the same value to all the cells in the grid.

- The default value which appears when you open "create grid data" tool, is used for filling the cells which can´t get the value from the shapefile. We can use a value or a "-99" in case we don´t want mohid to take into account them. For instance, if we were creating the soil map, and we had a shape with 2 different soils but these dont cover all our grid, we would write "default: 3" so that we could later define a third soil´s hidraulic parameters with mean values or so...

Right?

Thanks again,
ane

Jauch
Site Admin
Posts: 98
Joined: Thu Jan 01, 1970 00:00

Re: Grid data from a shape file

Postby Jauch » Thu Oct 20, 2011 11:19

Exactly :)

The only point I must clarify is that the -99 thing, to tell mohid that cell is not to use, only works in the topography file. But if you set a point to fill value inside de basin when you tells mohid land to delineate the basin, I'm not sure if it will work.

But for the soil grid data, the option to set to 3 that you mentioned is a good method. Of course, with a litle work, you can than edit the grid data in mohid studio and set these soils to one of the other two.

:)


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