4 Replies Latest reply on Sep 7, 2016 4:06 AM by Boreak Silk

# Polygon and point maps – coordinates of a point changed when overlaid on a polygon map

Hi all,

I’m trying to overlay a point map on a polygon map but one of the points does not stay within the corresponding polygon boundary.

Attached file has two map layers (polygon and centroid). I created these two layers in ArcMap and all centroid points are within the
corresponding polygons. However when I brought them to Tableau one of the points, Small_Area_ID 4150, falls outside its polygon.

Its coordinates are -37.82649894830 and 144.91859794800 in the data table but when I brought into Tableau and mapped they changed

to -37.83462 and 114.91638 respectively.

Any idea why? How to fix this problem?

Boreak

• ###### 1. Re: Polygon and point maps – coordinates of a point changed when overlaid on a polygon map

An educated guess:
The centroid as calculated by ArcMap is the actual centroid, that is, the spot where half the area is to the left of the point, half to the right, half above, half below. This is the average of the points of the polygon in a sense, but the average position of every point in the polygon, not just the vertices.

The calculation in Tableau is the average location of the vertices. To really see how this is different, imagine a triangle with one of the corners slightly curved, and that the curve is defined by 100 points to make it really nice and curvy. The other to corners are sharp, so are defined by just 1 point. Taking the average position of all of the points (as is happening in Tableau) means that the curved corner will be weighted 100x more heavily than the sharp corners.

A blog post that might provide some better solutions:

• ###### 2. Re: Polygon and point maps – coordinates of a point changed when overlaid on a polygon map

Hi Mary,

Thank for your response.

I was aware of the issue and used a function in the ArcMap to make sure the point is within the polygon.

And according to the XY coordinates, the point is in the polygon and when I mapped only the point layer

it is in the right position. However when I mapped both layers the point is off and I’m not sure why.

Kind regards

Boreak

• ###### 3. Re: Polygon and point maps – coordinates of a point changed when overlaid on a polygon map

Do you mean the point with ID 999?

That is not what you are plotting in Tableau. Click on one of the points, select the Show Data button in the tooltip, and then select the underlying tab. This is what is being plotted: the average latitude/longitudes, that is, the average of all of the the lat/long values that make up the polygon. If you want to plot just the point location with ID 999, you will need to follow the steps here:

Points and Polygons | The Last Data Bender

• ###### 4. Re: Polygon and point maps – coordinates of a point changed when overlaid on a polygon map

Hi Mary,

Thanks. That is exactly what I was after.

Kind regards

Boreak