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Dissolver Transformer not removing many Interior Lines

  • February 8, 2016
  • 3 replies
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Hi,

I'm using the Dissolver Transformer in FME Desktop 2014. I inputted a shapefile containing 34,000 polygons, expecting that I would see around 15,000 Interior Lines removed, however, only 23 were removed. Although I've specified that I need the Dissolver to group the inputs, there are plenty of adjacent polygons within each group that I would have expected the Dissolver to merge. I tried applying Coordinate Rounding to my input layer, but this didn't make any difference to the Dissolver output.

The Dissolver Transformer had originally looked as though it was working with my input layer, as I've run it twice already to reduce the number of polygons from 115,000 to 75,000, then from 75,000 to 34,000. However, the third iteration seems not to have worked. I'd appreciate any advice on what it might be that I'm doing wrong, and how I can try to fix it?

In case it helps to know the geographies, I'm using Scottish 2011 Postcode boundaries as my input, grouped by 2011 Data Zone. My objective is to split large Data Zones into 2 or 3 smaller parcels of land which nest exactly within the Data Zones, so I decided to use 2011 Postcodes as my building block for these parcels, as they nest exactly within the Data Zones.

Many thanks in advance for your help.

Kind regards,

Fliss

Best answer by david_r

Hi

Could it be that you have some very small gaps between your polygons? That could explain what you're describing. Rather than using the CoordinateRounder, try inserting a Snapper (type: segment snapping) with a small-ish tolerance and see if that helps.

David

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3 replies

david_r
Evangelist
  • david_rEvangelist
  • Best Answer
  • February 8, 2016

Hi

Could it be that you have some very small gaps between your polygons? That could explain what you're describing. Rather than using the CoordinateRounder, try inserting a Snapper (type: segment snapping) with a small-ish tolerance and see if that helps.

David

F
  • fliss
  • Author
  • February 9, 2016

Hi David;

Many thanks for your very quick response. You were spot on - I ran my input layer through the Snapper Transformer first, then used the output in the Dissolver Transformer and it did indeed remove far more Interior Lines.

I have unfortunately now realised that the Dissolver Transformer isn't actually going to achieve what I had hoped as I don't want to dissolve all the postcodes per Data Zone - I really want to be left with 2 or 3 polygons worth of merged postcodes which nest exactly within the Data Zones. I thought the Dissolver Transformer was similar to the Eliminate tool in ArcMap but with the benefit of allowing processing by group (Data Zone) to achieve the nesting. However, I've now realised it's just the same as the Dissolve tool in ArcMap (I know, I know, the clue is in the name)!

Thanks anyway though for your help, as it is still useful to know about the Snapper Transformer for future use.

Kind regards,

Fliss

david_r
Evangelist
  • david_rEvangelist
  • February 9, 2016
fliss wrote:

Hi David;

Many thanks for your very quick response. You were spot on - I ran my input layer through the Snapper Transformer first, then used the output in the Dissolver Transformer and it did indeed remove far more Interior Lines.

I have unfortunately now realised that the Dissolver Transformer isn't actually going to achieve what I had hoped as I don't want to dissolve all the postcodes per Data Zone - I really want to be left with 2 or 3 polygons worth of merged postcodes which nest exactly within the Data Zones. I thought the Dissolver Transformer was similar to the Eliminate tool in ArcMap but with the benefit of allowing processing by group (Data Zone) to achieve the nesting. However, I've now realised it's just the same as the Dissolve tool in ArcMap (I know, I know, the clue is in the name)!

Thanks anyway though for your help, as it is still useful to know about the Snapper Transformer for future use.

Kind regards,

Fliss

Hi

Good to hear the Snapper worked.

You might also want to look at the SliverRemover, it is quite similar to the Eliminate tool in ArcGIS.

David

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