topic Re: Slivers between polygons when zoomed in. in Forestry Questions

Slivers between polygons when zoomed in.

ShanePrice — Thu, 05 Oct 2017 17:59:57 GMT

I've come across an issue when using clip. Sometimes when I clip a polygon to another polygon, very tiny slivers are created between the two polygons. These slivers are so tiny that they aren't even visible at a 1:1 scale. I need to zoom in closer to actually see them. I've attached some images to show exactly what I mean. I'm wondering if these slivers really do exist within the data or are they just a limitation of the software, because these slivers would be a fraction of a millimeter if measured in the field. . I've using ArcMap 10.4 right now.

Re: Slivers between polygons when zoomed in.

MicahBabinski — Thu, 05 Oct 2017 19:14:56 GMT

Hi Shane,

Have a look at this doc, specifically the section on XY resolution:

Feature class basics—ArcGIS Help | ArcGIS Desktop

Basically, your XY Resolution defines the precision with which you are storing your features. The default is .1 millimeters. So, what I think is mostly likely going on is that when you are clipping your polygon, ArcGIS is [metaphorically] saying "I can show Shane these clipped features, but the degree to which those feature edges are aligned is limited by the XY resolution of the feature class, so they can't be closer together than .1 millimeters" assuming you used the default.

Here's an example:

I've clipped the purple layer to the red layer. I'm zoomed way in. As you can see, the gap between their edges is less than .1 millimeter. Since both feature classes use the default resolution, ArcGIS can only draw those layer edges with alignment within .1 millimeters without changing the underlying data.

It's a bit confusing but hopefully that clears it up.

Micah

Re: Slivers between polygons when zoomed in.

DanPatterson_Retired — Thu, 05 Oct 2017 19:49:39 GMT

A couple of things...

is the file a featureclass in a geodatabase or a shapefile? (ie the whole topology issue)
does the file have a defined coordinate system (long story, but it has to do with geometry errors)
have you tried to 'fix' it using the Integrate tool to see if it is indeed 'fixable' or imaginary

Re: Slivers between polygons when zoomed in.

ShanePrice — Thu, 05 Oct 2017 20:08:08 GMT

Hi Micah,

Thanks for that explanation. That makes total sense and I can now sleep tonight haha. Now to explain this to everyone else in the department.

Thanks again.

Shane

Re: Slivers between polygons when zoomed in.

ChrisDonohue__GISP — Thu, 05 Oct 2017 20:21:52 GMT

I've had similar observations and talked with ESRI Support about it. Essentially what is going on is that at a scale of less than one to one the software can no longer visually display the linework correctly. So what is being viewed is not really an accurate depiction.

Of course, seeing these apparent slivers (and gaps) can be a bit disconcerting. Visual review is one way people see issues, so the question then becomes, is the issue real or not?

So here is one way to check. Measure the gap with the Measure Tool. As mentioned by Micah Babinski the XY Tolerance controls how close linework is to be considered coincident. If the distance measured turns out to be less than the XY Tolerance, then the linework is coincident, even though way zoomed in it looks like there is a problem.

Also, more generally, if one sees issues, check the scale right off the bat. If it is less than 1:1, realize that what is being displayed may not be an accurate depiction.

For more specifics, check out the reply from ESRI Support when I posed similar issues I noticed to them:

As per our phone interaction, a map scale past 1:1 is a known limitation of our software because at a scale of 1:1, the screen display is equal to the actual distance of objects on the earth's surface (i.e. an inch on your screen is equal to an inch on the ground). When you go past that scale, it may appear that features are overlapping (when in reality) what you are seeing is the actual software resolution tolerance pushed past it's maximum threshold. In essence, the entire screen is only one point (at a map scale of 1:0), which is why your work flow is producing inaccurate results.

Strangeness - gaps unintentionally created while editing

Chris Donohue, GISP

Re: Slivers between polygons when zoomed in.

ShanePrice — Thu, 05 Oct 2017 20:39:50 GMT

Thanks Chris. That made it even more clear. I measured the gaps and they're smaller than the XY Resolution so they're really just non-existent gaps. I'll just have to stop myself from zooming in so far so that I can just forget about them.