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I'm creating a map layer that supports up to zoom level 14 and I want to know what resolution US Census blocks are?

At zoom level 14 each pixel is approximately 16 meters.

I am wondering if I can simplify the block data to 20m resolution or if it is worthwhile to go with a higher resolution?

  • What client application are you basing your zoom level from? – artwork21 Oct 11 '13 at 20:17
  • Google Maps API. – Aaron Kreider Oct 11 '13 at 20:32
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(Normally we would just consult the Census Web site for metadata, but it's down for an undetermined time...)

Census features derive from the TIGER files developed over the last 20 to 30 years. These files were assembled from multiple data sources, primarily the USGS 1:100,000 Digital Line Graphs and 1:24,000 USGS quadrangles. The accuracy of these data was improved systematically during the last 5-10 years, culminating in a recent test against GPS-derived "ground truth points":

The test verified that the spatial accuracy of the street network met the Census Bureau's horizontal spatial accuracy standard of CE95 at 7.6 meters (about twenty-five feet) or better. This accuracy standard requires that 95 percent of the time, the distance between the sample control points coordinates and their corresponding street centerline file intersection points not exceed 7.6 meters, i.e., a file point will fall within a radius of 7.6 meters of its corresponding control point.

(http://www.scribekey.com/TIGERLineSpatialAccuracy.html)

It helps to recognize that these are tests of absolute positional accuracy. Relative inaccuracy would be reflected in distortions of true shapes and ought to be much, much better than 25 feet--but as far as I know, tests of such localized distortion (such as at the block level) have not bee reported.

Notice that scale is related to accuracy through its effect on precision: the larger the scale, the greater the precision and the more stringent are the accuracy requirements (as spelled out in the National Map Accuracy Standards, which roughly indicate that maps are expected to be accurate at most locations to within 1/2 to 1 millimeter on the map. At 1:100,000 scale, 1/2 mm = 100000*1/2 mm = 50 meters on the ground and at 1:24,000 scale, 1/2 mm represents 12 meters.) "Resolution" is an irrelevant concept, because the vector-based representations of these data consist of (lat, lon) coordinates in double precision, which has more enough precision to locate the nuclei of individual atoms.

We can nevertheless estimate some reasonable bounds on resolution from this information. For instance, representing Census blocks as an image with 20m pixels would introduce more imprecision into the shapes than is inherent in them, given the 7.6 m absolute accuracy: one can safely render blocks with sub-7.6 m pixels. Indeed, there is a risk of losing some of the smallest blocks altogether at a 20 m resolution. How much more resolution might be worthwhile to use will depend on the application and whether absolute positional accuracy or just relatively accurate rendering of the block shapes is needed. It also depends on what additional geographic data sources might be overlaid or combined with the blocks: it would make little sense--and be computationally wasteful--to use a resolution that is substantially finer than the accuracy inherent in the lowest-quality data source.

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I don't know the exact resolution, but having worked with them I know they represent actual blocks of streets and are at a finer resolution that that.

That being said, if all you are doing is looking at them, sure, simplify to the max zoom level you'll view at. If you are going to execute spatial queries, though, I would not do that as you will create gaps that will cause occasional queries to miss a block (i.e. a map click at what looks like the edge between two of them may not hit either.)

  • What is the "resolution" of "actual blocks of streets"? – whuber Oct 13 '13 at 16:19
  • I didn't mean that there is a "resolution" of blocks, but census blocks are actual single street blocks and at zoom level 14 typical street blocks in San Francisco are about a centimeter long on my monitor. I've worked with census data and while it doesn't seem to target a specific scale, it is pretty detailed. I don't have blocks handy but I have block groups and I can visually see that they have detail that is lost at ZL 14. – Russell at ISC Oct 14 '13 at 13:53
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Think about it from a different perspective. The blocks are in part delineated by: state, county, tract, block group, voter district, school district, city, TAZ, PUMA, and major road. Each block uniquely separates each other level of geography, and the accuracy of the block's boundary is dependent on the accuracy of their boundary. When looking at it from a smaller level, you have to take into account the accuracy of the borders delineated by roads and streets. Since there is much issue over the accuracy over the position of roads assigned by the Census Tiger shapefiles, it would be reasonable to expect that the inaccuracy would be reflected by the Census Blocks as well.

You can probably trust the Census boundary assigned for states, counties, and cities. But there would be some issue for tracts and blocks.

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