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Output resolution of vector data from ArcMap

November 10 2008 | 0 comments
Categories: Publishing

In ArcMap's Export Map to Illustrator, why are x,y data quantized? For example, at resolution = 600 dpi, x,y data are rounded to the nearest 1/600th of an inch. This seems needless - see attached PDF for a fuller description of the bad effects that are possible. Thanks in advance for your response.

Mapping Center Answer:

The quantization is a limitation of ArcMap's display and output subsystem. Our export process works as follows:

  1. Draw the given content to an EMF using a device context from the default printer at the selected resolution.
  2. “Play” this metafile into a converter which then writes out the requested format (AI, PDF, JPEG, etc.) and writes it to disc.

The first step is the reason you are seeing some grid-fitting. All of the drawing for display, output, or printing in ArcGIS Desktop utilizes the Microsoft GDI (Graphics Device Interface) API. Any “Draw” request from a graphical object must be given a “device context” which to draw; this inherently includes the dpi and size (in pixels) of the “canvas” to which it must draw. It is this “canvas” specification that dictates the grid to which the data is written. That is why you must specify an export resolution, and why this resolution affects the placement of points in your vector data.

To address the question of why not just “scale the x/y data and output the results unaltered” - the answer is that GDI only utilizes an integer coordinate system. If the scaled coordinate falls between one integer and another, then the point will be snapped. As you have noticed, requesting output at a higher resolution will reduce this effect because less snapping is necessary.

A side note: if the symbology of the data dictates that it must be rasterized, rather than being retained as vector data, then that layer and all the layers below them are drawn into a raster image and blended together to retain the transparent or rasterizing symbology (picture fills/markers, etc.). This will of course result in some “grid fitting” at a resolution determined by the resolution specified for the export in conjunction with the Output Image Quality ratio selected on the export dialog.

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