Combining objects based on another layer

[MatthiasMeynen]

I'm trying to combine a building layer and a parcel layer. The goal is to combine all the buildings on one parcel to one object. The only relationship the buildings and the parcel have is spatial. When I use spatial join the parcel gets combined with one building, and this for all of the buildings on the parcel separately.So the question is, how can I combine the different buildings on the same parcel? Already thanks for the help!

[OllieBrown]

Hi Matthias,

This sounds like something you could use the intersect tool but I'm unsure without a bit more information. Do you mean that you want each building to have the attributes of the larger parcel? Also have you changed the order that you are joining the buildings to the parcels in, for instance.... try joining the parcel layer to the buildings and vice versa and see if this gives you the output you're after.

Cheers,

Ollie

[MatthiasMeynen]

Thanks for the reply already. The ultimate goal is that I want to calculate the overlap with a flood map for all the buildings on the same parcel in 1 measure (in percent). I also want the buildings to have the identification number from the parcel. If you need any more info, just let me know.

[OllieBrown]

OK so spatial join by location should work then, right click your building layer and choose joins and relates > join. Use the drop down at the top of the dialogue to change from an attribute join to a spatial location join. Then select the layer you want to join them to and select the second radio button down to make sure each polygon is given all of the attributes of the polygon it falls inside, as in the attachment.

Once you've done this you'll have a feature class or shapefile with the attributes of both polygons, you can then sum the area of the buildings per polygon by doing a dissolve based on the polygon ID within your building layer and divide this figure by the area of the flood map polygons. Times this figure by 100 to work out the percentage of floodplain covered by buildings. You can do this by adding a field in the attribute table to do your calculations in or you could export the tables and do it in excel, up to you I guess.

Hope that helps.

Ollie

[MatthiasMeynen]

Ok those tips seem to be working pretty well. So thank you already. But I would also like to know the overlap with the flood map for each polygon separately. Not just the total overlap. Any ideas how I could do that?

[OllieBrown]

So you mean if a building polygon is totally in the flood plain then it would be 100%, if a quarter of it was it'd be 25% etc?

[MatthiasMeynen]

Indeed. I want to compare with data from households collected by surveys to validate the flood map

[OllieBrown]

Hi Matthais,

I've attached a word doc with a very quick run through on how to do what I think you're after. Like I said it was pretty quickly thrown together so if anything doesn't make sense give me a shout and I'll clarify it.

Hope it helps.

Ollie

[MatthiasMeynen]

Ok I'll check it first thing next week (don't have acces to Arcgis this week anymore) but I'm already very very very thankfull!

[OllieBrown]

OK mate, hope it goes well. You're welcome too, pay it forward an all that haha