How do I determine area of multiple polygons within a Land Parcel Polygon

[Dan Burrell]

Hi All

 

I am still quite new to GIS concepts but have been mucking around with MapInfo for a while.

I have an interesting question.

 

I have two sets of data.

 

1) One which is the land parcel outline for land parcels in my district ( And land areas )

 

2) Another which is forestry areas in our district. (and Area of each polygon )

 

 

I have determined hectare areas for both polygons.

I am looking to find out which land parcels have a forestry area greater than 20 hectares.

 

 

I can determine which forestry polygons are 20 hectares and over, but how would I go about determining if multiple polygons added UPTO 20 hectares... within a land polygon.

 

I am not scared to do some Map Basic if need be.

 

Screenshot shows my problem. Multiple Polygons (black) inside land parcel (green)

 

Any suggestion or help would be amazing

 

thanks!

 

 

 

 

[Kannan Krish]

I

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[Kannan Krish]

i would first update the area of the polygon in a column, and do thematic maps based on their area measurement, hence will get what i want is that you are trying to do?

[Dan Burrell]

Thank you for your reply.

Sort of. I do want to create a thematic map eventually.

First i simply want to be able to identify the total area of the forest polygons the lie within the land parcel.. (some overlap many parcels.. )
If the total for the parcel exceeds 20ha. I want to select it. Or colour it or whatever.

The land parcels are one table and the forest polygons are another table. So visually they do overlay.

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[Peter H]

Hi Dan,

SQL select will be your friend........
Let's assume you have a unique identifier for each land parcel. If
not, you will need to create one.

Under Query - SQL select

Fields {*}
Tables {Fparcels, Lparcels}
Where {Fparcels.obj within Lparcels.obj}
Group {}
Order {}
Into {FPLP}

This will assign the LP unique id to the FP.
Then using SQL select again

Fields {Sum(FP_area), LPid}
Tables {FPLP}
Where {}
Group {LPid}
Order {}
Into {Selection}

This will now show LPid with the sum of the forestry area within them.
Just select the ones that are 20 ha and greater

Good luck
Pete

>  20 hectares .jpg
> 26KViewDownload

[Thomas Bacon]

Hi Dan,

Peter's response will work assuming that all Forestry areas are entirely contained within a land parcel. As this is most likely not the case, I would suggest you have a look at the ProportionOverlap() function. You will probably need to loop through your parcels, select any intersecting forestry areas and then loop through these returning the proportion overlap for each and adding it to the previous value (cumulatively summing the overlapping areas). There may be a better way to do it purely with SQL, but this is an option.

Regards,

Tom Bacon
GIS Engineer, Mouchel
T 01444 472380 │ E thomas...@mouchel.com │ W www.mouchel.com
Our values: innovation │ excellence │ integrity │ responsibility

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[Lars I. Nielsen]

SQL Select is indeed your friend.

But I would utilize the functions Overlap(), and possibly AreaOverlap() and/or ProportionOverlap(), to get the result.

And use "Intersects" between the regions in the two tables.

The functions are documented in the MapBasic reference manual.


Kind regards / Med venlig hilsen
Lars I. Nielsen
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-----Oprindelig meddelelse-----
Fra: mapi...@googlegroups.com [mailto:mapi...@googlegroups.com] På vegne af Peter H
Sendt: 16. april 2013 08:09
Til: MapInfo-L
Emne: [MI-L] Re: How do I determine area of multiple polygons within a Land Parcel Polygon

[Dan Burrell]

Hi All

 

Thanks for your amazing replies there guys. I really appreciate it.

You guys were onto an answer very quickly!

 

I did a bit of trawling around to see how to select the objects. I see the help actually has some useful pages around these operators.

 

I have come across this problem in particular and I dont understand what it means in relation to the intersects operator. ( attached screenshot)

 

There was some suggestion that my unique identifer fields had to be interger ( which they are now...) and indexed, which I have done.

 

I googled and couldnt find much help.

 

Any ideas?

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[Timothy Mashford]

I think it's a cryptic way of MapInfo telling you that you can't have a hyphen in your results name. Change it to e.g. Land_Forestry

[Mats Elfström]

Yes, the hyphen is treated as a minus operator. As a rule, avoid hyphens in file, folder, table, field names and variables. Eventually, they will cause trouble, even when they are not rejected at once. 

Hälsning / Regards

Mats.E

Skickat från min / Sent from my iPhone, Ursäkta att jag är kortfattad / Excuse my brevity.