How to display the real-world scale value when printing Web Mercator maps using the VertiGIS Studio Printing Engine

Introduction to Web Mercator and scale values

The Web Mercator projection is the default spatial reference used in ArcGIS web maps. The scale for this projection is accurate at the equator, but as you move farther away from the equator, the map is stretched to preserve direction. This is useful for maps that cover large areas or that are intended for international use, but the consequence is that distances are distorted, especially at high latitudes. As a result, the scale value you see displayed may not be the true, real-world scale.

VertiGIS Studio Web, Printing, and Reporting adjust the scale bar to compensate for the Web Mercator distortion, meaning the scale bar will be accurate wherever you are in the world, but the scale value (i.e. the number) doesn't get adjusted, and is treated more as a length-on-display. This makes it so that the value stays consistent as you pan around the map, but can't be used for exact measurements in most parts of the world.

What if I want my prints to show the real-world scale value?

In some cases, the scale distortion from Web Mercator may pose some challenges. For example, if you wanted one inch on your printout to equal exactly 100 ft. in the real world, you might think to set the scale to 1200. However, because of the stretching of the map at latitudes away from the equator, the result would likely be zoomed in closer than expected, meaning one inch on the printout would be less than 100 ft. in the real world. Again, the scale bar on the printout would still be accurate, but measurements and calculations based on the scale might become more challenging.

One possible solution is to use a local projection as your web map's spatial reference. This shifts things so that both direction and distance are accurate within the relatively small geographic area where you are located. However, this can be challenging since it means you need to create and maintain your own basemaps in the local projection, and may also require you to re-publish your data in the new projection.

Another possible solution is to manually perform the calculations to adjust for the Web Mercator distortion (available under "Step 1: Find the correct ratio to use" of this article) and apply them to the scale value in the print settings whenever you run a print. However, this either requires your users to have a sufficient amount of technical/mathematical knowledge to find the adjusted scale value themselves or means you need to invest time into developing a workflow that performs the calculations for them.

Solution

If you are using the VertiGIS Studio Printing Engine and are on VertiGIS Studio Web version 5.35 or newer, you can take advantage of datum transformations to automatically adjust the projection of the print so that the scale is accurate to your location.

To set this up:

1. Open your app in VertiGIS Studio Web Designer. (NOTE: Make sure the app is configured to use the VertiGIS Studio Printing Engine to run prints.)
2. In the right panel, click on the Services tab and select Coordinates.
3. Under Datum Transformations, click the "+ Add Transformation" button.
4. For the Input WKID, enter 102100, which is the well-known ID for Web Mercator.
5. For the Output WKID, enter the well-known ID for your local projection. For example, Victoria, BC might use 3157. (NOTE: It's important to remember that the local projection will only be accurate within a small geographic area, so you want to choose the one that is most appropriate for your region.)
6. Click Submit and save the app.

When running the print:

1. Click the Print button or whatever you have configured in your app to open the Printing component.
2. Enter the desired scale value into the Scale parameter of the print options. (NOTE: This will lock the print area, so make sure you have panned the map to the location you want to print, first. If you want to move the print area, toggle off the Lock option.)
3. Expand the Advanced Options section and use the Projection dropdown to select the local projection you configured earlier. You should see a larger bounding box appear on the map, which represents the true-to-scale print area.
4. Click the Print button.