How to calculate percentage of a study area for land cover change?

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04-07-2018 10:28 PM
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New Contributor II

I'm trying to calculate percentage of the study area that's under land cover change from 2001 to 2011. Would I add a new field and then go to field calculator and divide NLCD2001 and NLCD2011 and multiply by 100? Or am I missing a step and have to do something else instead?

Does anyone know how I should do this? Thank you

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DanPatterson_Retired
MVP Esteemed Contributor

Newest date value minus oldest date value is the difference that occurred over time... time moves forward.

This difference, divided by what it was, (expressed as a percentage) is the change that occurred over time

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MichaelGinzburg
New Contributor III

Hello,

You are correct regarding adding a new field and calculating with the Field Calculator.

The whole process:

How To: Calculate the percentage of area for polygons 

Add another new Double field, 'Percentage', and run the Field Calculator where 'OrigArea' is the original Area field from the 'county' layer:

[OrigArea] / [InterArea] * 100
__17
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New Contributor II

Okay thank you! So in this case based on the information in my attribute table, would I plug in [NLCD 2001]  / [NLCD 2011]  * 100 in the field calculator?

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AbdullahAnter
Regular Contributor II

If the values in your fields ( NLCD2001 and NLCD2011) represent the area of land cover. The percentage of change should to be calculate as the following:

( (NLCD2011 - NLCD2001)   /   NLCD2001 ) * 100

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__17
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New Contributor II

Okay thanks! I'm wondering why did you plug in NLCD2011 before NLCD2001 if the order is the opposite in the attribute table? Also, why did you divide it by NLC2001? Could you explain this for me please? I'm just trying to understand all of this. Thanks again. 

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DanPatterson_Retired
MVP Esteemed Contributor

Newest date value minus oldest date value is the difference that occurred over time... time moves forward.

This difference, divided by what it was, (expressed as a percentage) is the change that occurred over time

View solution in original post

AbdullahAnter
Regular Contributor II

Y2011 - Y2001 = the change in ten years period

and this value will divide  by  Y2001 to know the  change value relative to the previous status (Y2001)

finally multiply by 100 to get the change percentage % within 10 years

Also you can divide all that by 10 to get yearly change percentage.

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__17
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New Contributor II

I entered that equation you mentioned and it shows these numbers as the percentages in the attribute table. Is this correct? Like are these percentages supposed to be very high?

Thanks

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__17
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New Contributor II

Because I'm trying to get the percentage of the total study area under land cover change.

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AbdullahAnter
Regular Contributor II

hmm.. you will do some thing else.

you will add two fields one for year2001 and the other for year 2011

then in Year 2001 field  you will divide  NLCD2001 by the total of study area ( use statistics for COUNT field and get the Sum  ).

then do the Previous step with new field Year 2011.

the Equation will be same but change to new fields

( (Year2011 - Year2001)   /   Year2001 ) * 100

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