Occasionally, when knocking up a demo or a quick project in ArcGIS, you may find that your scale bar is lying to you. In the majority of cases, most GIS users don’t even take notice of their scale bars, much less question the veracity of the distances implied. However, if you have a projection or units disagreement, the scale bar will be noticeably incorrect. There are two warning signs that this is happening to you. First, the scale will be wildly incorrect – a distance of 500 feet might read as five miles, for example. And second, there will be unnecessary decimal precision in the scale bar’s divisions. New or inexperienced users may feel frustrated by this, because ArcGIS (as of version 9.3) does not allow the user to set the actual range of the scale bar. It might make sense to force ArcGIS to produce a four inch scale with one-inch and half-inch divisions, which would make the numbers nice and even – but then the scale bar couldn’t scale if the scale is changed. So the ranging of the scale bar is better left to automated and dynamic processes when working on the Desktop.
Figure 1: Data frame properties dialogue box, ArcGIS 9.2
Fortunately, this problem is easy to fix. It’s not actually a scale problem: it’s a units problem. Take a look at the ‘General’ tab of the Data Frame Properties dialogue box (figure 1). There are two different units that need to be set: the map unit, and the display unit. If all of your layers are projected correctly, the map unit will be auto-filled, and the dropdown will be inactive (grayed). So the first thing to do is make sure your projections are squared away. In most cases, making sure all the layers are being projected correctly will solve this problem. But occasionally, you may end up working with a set of layers that doesn’t have a projection. If none of the layers has a projection, the map unit will be set to ‘Unknown Unit’, and the dropdown will be active. This problem can be easily remedied by selecting the correct map unit from the dropdown. Then you can set your display unit to whatever makes sense for the project. Generally, it is best to keep the map unit and the display unit the same. Different units make comparing measured distances with actual distances difficult, and can lead to general confusion.
What happens when you don’t know what the map unit is? The first thing to do is look for metadata. Metadata should include the measurement unit for every field in the dataset, including any spatial layers. However, if you don’t have a projection, you probably don’t have any metadata either. In that case, the best you can do is guess. I’d start with a roads layer, if you have one. Each segment of the road network will have a length available. US datasets will generally contain lengths in feet and areas in square feet. Assume that the length is in feet, and then convert that value to miles. Does that make sense? If it does, give feet a try. If it seems three times too large, then the map unit is probably meters. Eventually, you’ll find the right unit.
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