How can you tell what map scales are shown for online maps?

By Aileen Buckley, Mapping Center Lead

Zoom Levels Thumb

As you zoom in (or out) of the online maps you see on Virtual Earth (VE) or Google Maps (GM), you are actually seeing a series of different maps with slightly different information displayed at each zoom level. Zoom level is indicated and controlled in an online map by the vertical zoom slider, like the one shown at the left in the image here. Whenever the zoom level is changed, a different map is shown.

Of course, these maps are well designed so that viewers are largely unaware that they are seeing these different maps. The foundation for good design of an “online map” hinges on understanding how to design for each of the zoom level represented in the entire online map. Colors, fonts, number of and types of features, etc. are all seriously considered when each of the maps is created for each of the zoom levels.

When authoring this kind of online map with ArcGIS, a map document containing group layers, one for each zoom level, is a good approach. (The Working with layers and scale ranges blog entry provides a good overview of how to organize a map document this way.) Each zoom level in the online map is represented by your work at a specific map scale in the ArcMap document. The hard part is to figure out which zoom level matches to which map scale.

There are twenty zoom levels for Virtual Earth or Google Maps. The corresponding map scales that you would design and create your maps at if you wanted them to mash up on VE or GM are:

Scales Table

For some months now, we’ve kept printed versions of the table above taped by our monitors so we could easily remember which map scales matched the zoom levels.

When you create a cached map service, tiled images are created at each map scale from your map document. These images are stored in a pre-defined folder structure on your server, and these folders are named for the corresponding zoom level.

Folders

Knowing which map scales match the zoom levels is important because sometimes caching a map takes a long time. If there’s any error in symbology or labeling settings for a certain zoom level, that map scale will need to be re-cached, which could take a lot of time. By browsing into the folders where the cached image tiles are stored, you can preview the tiles and verify that they look good. If they don’t, cancel the caching process, fix the map document, and re-start the caching process.

In order to help with creating maps at these zoom levels easier, use this cachescales.txt file – it can be loaded into ArcMap’s scale control when you choose to customise the list (with Version 9.3). Setting your available map scales this way, it is easy to work at exactly the map scales users of your map service will see as they zoom in and out of your online map.

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9 Comments

  1. dewright_ca says:

    So you have this cachescales.txt file, do these correspond to match the scales used for ArcGIS Online? Or are these another internal scale system?

  2. abuckley says:

    The scales in the cachescales.txt file match the Microsoft Virtual Earth / Google Maps tiling scheme scales.

    The .txt file for the ArcGIS Online cache scales is located in: C:InetpubwwwrootArcGISManagerHelparcgis_online_scales.txt as described in the online help at: http://webhelp.esri.com/arcgisserver/9.3/dotNet/index.htm#designing_overlay_agol.htm.

  3. stacemaples says:

    I’m creating an ArcMap template with Group Layers and scale ranges for each of the Google/VE scales, so that I can just drop layers into each of the Group Layers when making maps for the JSAPIs. I’m using the Google VE map scales text file for this, but I’m noticing that, although in the txt file the scales have several decimal places of precision, they are displayed in the ArcMap Map Scale dropdown without this precision. Additionally, when I set scale ranges in the properties for each Group Layer, the available scales do not have the precision of the actual txt file. I have found that, for several scales, I have to decrease the numeric scale to make a layer render at its corresponding GVE scale. I suspect this is being caused by the difference between the way ArcMap is scaling the map doc from the Map Scale dropdown (using the precise scales, even though they are rounded in the dropdown list), while the scale range tools in the Group Layer properties are using the scales as displayed in the dropdown, without the precision.

    OK… three questions…1) is this going to cause problems when generating tilecaches to overlay in Google and VE? 2) If so, should I manually enter the precise scales from the txt file into the scale range options in Group Layer properties? 2) Is this a bug that should be reported?

  4. marylandgis says:

    My question is a bit esoteric, but here goes:

    Map scales on paper, of course, represent the relationship between the same unit of measure on the paper and on the ground. How does the apply to computer screens? Is there still a formula that runs from ground measurement through screen resolution (pixels) to distance on my screen? So that, theoretically, I could place a wooden ruler over my screen to measure distances (If I know the scale and can do the math)?

  5. abuckley says:

    Dear dewright_ca:

    Yes – the cachescales.txt do relate to the scales used on ArcGIS Online.

  6. abuckley says:

    Note that with ArcGIS 10, you can load these scales directly in ArcMap! Just click the down arrow for the map scale and then click Customize This List. From there, if you click Load and select the option for ArcGIS Online / Bing Maps / Google Maps, these map scales will be loaded for you!

  7. abuckley says:

    Dear stacemaples:

    You do not have to be that specific in ArcMap — all you have to is to make sure that the scale that will ultimately be used (the one in the list above) is within the range of scales you set for the group layer. For example, if you are working with the 1:1,128 etc… scale, then you can set the scale range from 1:1,000 to 1: 2,000. When you cache the map, the tiling scheme will automatically use the precise number in the list. This makes your ArcMap work easier and you are still assured that it will cache correctly. All you have to do is make sure that for each group layer only one of the cache scales falls within the scale ranges.

  8. abuckley says:

    Dear marylandgis:

    While your question is a bit esoteric, we still love it! And in fact have wondered the same thing ourselves. So we blogged about it! Take as look at Kimerling’s “Map Viewing Altitude” blog post to read the answer to your question: http://blogs.esri.com/Support/blogs/mappingcenter/archive/2008/02/25/map-viewing-altitude.aspx

  9. TomPF says:

    So given the Zoom Level/Map Scale table above, if you were using one of these map services in ArcMap and your map scale was set to 1:2,000, which LOD tile would be showing, Level 18 or 19? I’m unclear how to interpret this table to determine where a particular level starts and ends.