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#1
josie

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Hello,

Happy new year everyone! Well, new year = new problems in arcmap...so here goes...

I have a map which I created. On the map is an area, and around that area I created a multiple ring buffer with the distances 1;2;3 nautical miles. However, I then in layout mode added a scale. The scale, when I measure off 1nm is not equal to 1nm on my buffer. Does anyone know if this is merely an unavoidable projection problem or whether I have done something wrong. The scale bar in in nautical miles and is off by a couple of millimeters (on my ruler).

Thanks again.

Josie

#2
François Goulet

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It could be a projection problem. What is it?

#3
josie

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It is in WGS 84. The buffer actually looks to get slightly thinner around the sides.

#4
josie

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sorry my mistake, it is in mercator projection infact.

#5
François Goulet

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sorry my mistake, it is in mercator projection infact.


If I'm not mistaken, the Mercator projection preserves angles and directions on the map (which is important for nautical charts and maps), but it is not equidistant so distance and scale will only be true along the equator or secant latitudes.

I'm not the projection expert so no other comes in minds, sorry!

I'll keep searching.

#6
josie

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That would make perfect sense actually. It is definate that the map is infact correct, I have measured, however the scale bar is the part which is wrong. I have never noticed it before really.

#7
Hans van der Maarel

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That would make perfect sense actually. It is definate that the map is infact correct, I have measured, however the scale bar is the part which is wrong. I have never noticed it before really.


The case with Mercator, as Francois has already mentioned, is that it's not equidistant. So the scale differs by latitude (and yes, the scale bar is probabely correct, but only for one specific latitude).

Your options are:
  • Find out which latitude that is and list that in the legend
  • Remove the scalebar

Depending on your audience and the purpose of the map, I would suggest #2 to be on the safe side.
Hans van der Maarel - Cartotalk Editor
Red Geographics
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#8
josie

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My base map is most of the North sea Europe area, however in my layout I am only offshore of the Netherlands (near Maas Noord). Am I correct in thinking that for small distances is shouldn't be too much difference but for larger distance it would be more noticeable? Further, would you class this as a big area as the base map is fairly large? Sorry if this is confusing. I very much appreciate the help. I agree with what has been said here, I guess I am just more intrigued than anything now for future reference.

#9
Hans van der Maarel

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My base map is most of the North sea Europe area, however in my layout I am only offshore of the Netherlands (near Maas Noord). Am I correct in thinking that for small distances is shouldn't be too much difference but for larger distance it would be more noticeable? Further, would you class this as a big area as the base map is fairly large? Sorry if this is confusing. I very much appreciate the help. I agree with what has been said here, I guess I am just more intrigued than anything now for future reference.


In that case, since your area of interest is fairly small, I would recommend looking for a different projection. Equidistant conic for example, use a central meridian right through your area of interest and standard parallels at roughly 1/4 and 3/4 of its north-south range.

If the area is close to the Dutch coast, you could simply use the RD system.

On small areas the differences in length on a Mercator projection are small, but they are certainly there. If somebody in your target audience takes a ruler, measures a distance and then makes decisions based on those distances, they're misinformed, which could have all kinds of consequences.
Hans van der Maarel - Cartotalk Editor
Red Geographics
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