Bug
The DotDensityRenderer in ArcGIS Maps SDK for JavaScript does not accurately represent the total population of a polygon at large scales when setting the dotValue equal to one.
| Bug ID Number | BUG-000163492 |
|---|---|
| Submitted | December 6, 2023 |
| Last Modified | December 6, 2024 |
| Applies to | ArcGIS Maps SDK for JavaScript |
| Version found | 4.28 |
| Operating System | Windows OS |
| Operating System Version | N/A |
| Status | Known Limit |
Additional Information
This is a known limitation by design in the DotDensityRenderer. Dot density visualizes the approximate density of a population and is not intended for visualizing exact counts. This is true whether the dotValue is equal to 1 or any other number. For example, if a polygon has a population value of 351, and the dot value is 100, how many dots is supposed be rendered in the polygon, 3 or 4? Neither will technically be precise. Similarly, if the dotValue is 1, then the number of dots should be 351. However, in practice, the actual rendered number of dots is an approximation. There is an approximation because dot density is calculated using a probability matrix given the pixels used to represent an area. For example, if a polygon takes up 1000 pixels on a screen and the dotValue is 1 and the data value is 11, then a threshold is determined such that for each pixel, when assigned a random number, the probability that about 11 pixels are rendered in the polygon is very high. Since it is based on probability, there is a margin of error. Some polygons may have a few extra points; others may have fewer. In the case of demographic mapping, that margin of error in dot count is OK, because demographics have a margin of error and some mathematical noise built in by design. In short, the dots in a dot density renderer are not to be taken too literally in either their placement or the number within each polygon. Users are not expected to physically count the dots on the map to know the value of an area. The dot visual communicates the pattern, and the popup or label can reinforce that pattern with the actual value. In a lot of ways, a dot density map is not too different from a choropleth map. It is highly unlikely that the actual hue of the shape tells the exact value. It helps to establish how the area fits into the pattern of the data, while the label or popup communicates the exact value. The team plans to update the documentation to make these points clearer.Steps to Reproduce
Bug ID: BUG-000163492
Software:
- ArcGIS Maps SDK for JavaScript
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