Map scale and scale bar

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11-05-2021 02:17 PM
StephenKing3
New Contributor II

Hello,

I'm looking for some advice. If I use WGS 1984 as my map coordinate system, is it appropriate to show a scale bar and a scale (e.g. 1:450,000) on the map? My understanding is that a fixed scale bar wouldn't be appropriate as you can't take accurate linear measurements from an unprojected map. Is it also not correct therefore to show a scale either?

Thanks.

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HeatherSmith
Esri Contributor

@StephenKing3 I wasn't sure of the answer to this one, so I asked John Nelson, who recently wrote about a similar topic (which was possibly the inspiration for your question). Here's what he had to say:

"In Pro, the scale of a layout (so its reference scale and any scalebar you put in a layout) corresponds to the center of the map. I think the answer I would give in this case is, ‘it depends but leads towards sure.’

Some of the dependencies that come to mind:

The latitude where this person is mapping will have an impact on how inconsistent the scale would be from the top vs the bottom of the map. More extreme latitudes will amplify it. Just a quick visual spot-check in the northern USA the scale distance difference was imperceptible to my crusty eyes.

At the scale they are mapping, (1:450,000) the scale difference across the map is negligible. If it’s an engineering schematic where a map reader is actually using the scalebar to do precision planning and measurements I’d say no. But I doubt that’s the case.

They can always add a small text note next to the reference scale and scalebar on the layout indicating that the scale corresponds to the center of the map and there can be minor, probably imperceptible, differences in scale at the top and bottom of the view."

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HeatherSmith
Esri Contributor

Hi @StephenKing3 

I recommend using a projected coordinate system instead of WGS 1984.

It's impossible to draw the earth on a flat map without a projection. So when you set your map's coordinate system to a WGS 1984, the map that you see in ArcGIS is still projected. It is drawing using the "Pseudo Plate Caree" projection. It's called pseudo, because it reads out the coordinates in latitude and longitude, instead of meters, but otherwise, it's the plate caree projection.

You can use a scale bar with that projection. However, I still recommend choosing another projected coordinate system that is more suitable for your map. Here's the information about plate caree distortions:

"The plate carrée projection is equidistant along any meridian and the equator. Shape, scale, and area distortion increase with the distance from the equator. North, south, east, and west directions are always accurate, but general directions are distorted, except locally along the equator. Distortion values are symmetric across the equator and the central meridian."

You can find some advice and tools for choosing projections in this lesson: https://learn.arcgis.com/en/projects/choose-the-right-projection/ 

StephenKing3
New Contributor II

Hi @HeatherSmith, thank you for your reply. I would normally use a projected coordinate system, however, in this case I have some boundaries that were digitised using WGS84 and they do not match the underlying boundary data when projected (a different issue). So in this case I have just created maps to show individual  features in WGS84. Is a scale bar accurate for a small area (maybe 2-3km wide) with the plate caree projection and should you show a scale? 

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HeatherSmith
Esri Contributor

@StephenKing3 I wasn't sure of the answer to this one, so I asked John Nelson, who recently wrote about a similar topic (which was possibly the inspiration for your question). Here's what he had to say:

"In Pro, the scale of a layout (so its reference scale and any scalebar you put in a layout) corresponds to the center of the map. I think the answer I would give in this case is, ‘it depends but leads towards sure.’

Some of the dependencies that come to mind:

The latitude where this person is mapping will have an impact on how inconsistent the scale would be from the top vs the bottom of the map. More extreme latitudes will amplify it. Just a quick visual spot-check in the northern USA the scale distance difference was imperceptible to my crusty eyes.

At the scale they are mapping, (1:450,000) the scale difference across the map is negligible. If it’s an engineering schematic where a map reader is actually using the scalebar to do precision planning and measurements I’d say no. But I doubt that’s the case.

They can always add a small text note next to the reference scale and scalebar on the layout indicating that the scale corresponds to the center of the map and there can be minor, probably imperceptible, differences in scale at the top and bottom of the view."

View solution in original post

StephenKing3
New Contributor II

Hi @HeatherSmith, thanks for coming back to me again - and for the advice and tips from John, that's all very useful.

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