topic Re: Setting the correct scale for a scale bar in ArcGIS Online Questions

Setting the correct scale for a scale bar

RuthHoban — Fri, 20 Jun 2014 08:20:50 GMT

I'm using two coordinate systems to carry out my project;
1. WGS 1984 for the analysis
2. WGS 1984 World Mercator to display the output maps

The WGS 1984 World Mercator scale is incorrect for my area and want to apply a WGS 1984 scale to a WGS 1984 World Mercator map.

Does anyone know if this is possible?

Many thanks,
R

Re: Setting the correct scale for a scale bar

JoeFlannery — Fri, 20 Jun 2014 10:41:19 GMT

Ruth:

How do you know that the WGS_1984_World_Mercator_Auxiliary_Sphere scale is incorrect? When I measure features with known dimensions I get the correct measurement using the AGOL measuring tool.

Where is "your area?"

How do you run an analysis in AGOL using GCS_WGS_84?

Re: Setting the correct scale for a scale bar

RuthHoban — Fri, 20 Jun 2014 10:45:57 GMT

Hi Joe,

I've measured the distances using the measuring tool then compared them to the distances measured in Google Maps. WGS_1984_World_Mercator overestimates the distance.
The area that I'm looking at is on the west Coast of Scotland. Is there another projection that I maybe able to use?

Thanks,
Ruth

Re: Setting the correct scale for a scale bar

JoeFlannery — Fri, 20 Jun 2014 11:28:50 GMT

Ruth:

Cool! I visited Scotland in 2000 and spent most of my time in the West of Scotland. Isle of Skye, Isle of Harris & Lewis, etc. Beautiful country!

Anyway, I created an ArcMap document, set the coordinate system to British National Grid and then I created a square polygon on top of the town of Ullapool that is 500m by 500m and exported it to a shapefile. I zippped the shapefile and uploaded it to AGOL. I zoomed to the polygon and measured it corner to corner in the east/west direction and north/south direction. I get 500m plus or minus one meter (mouse clicking accuracy) every time. See attached shapefile and give it a try yourself.

I'm fairly certain that the display coordinate system in AGOL cannot be changed from Web Merc. Do you think that it is purdent to be using Google Maps as your gold standard for accuracy of measurements? Do you have ArcGIS for Desktop (ArcMap) available at your desktop?

[ATTACH=CONFIG]34762[/ATTACH]

Re: Setting the correct scale for a scale bar

MikeMinami — Fri, 20 Jun 2014 15:25:24 GMT

When you add a layer, for some layer types, you can specify that you want it to display as the basemap. The basemap defines the coordinate system for the map.

As far as distance measurement goes, AGOL calculates the true distance and doesn't use the basemap as the reference. Web Mercator is not an equidistant coordinate system and would not return accurate results.

Thanks,

Mike