Why is exporting Image Service raster data in ArcMap so much slower than exporting the entire map document (which contains the image)?

Question

I'm trying to create a local copy of orthoimagery data (currently hosted as an image service) to improve performance in ArcMap 10.7 while I work in this map document.

To do this I go through the same process I would when exporting vector data from a feature service, such as parcel data published online by the state. I open the Export Data dialogue box (pictured below) by going to Table of Contents, right click layer > Data > Export Data. At this step I usually have to increase the cell size before it will let me export anything (and I may or may not adjust pixel depth/NoData).

Exporting this raster data, even just that within the data frame extent, into a TIFF takes 5+ minutes. Even if that data frame is perhaps only 500ft x 500ft. On the other hand, exporting the whole map document (layout and all), which contains that same image service as a layer in the data frame, to a PDF takes maybe 1-3 minutes.

How can this disparity exist if ArcMap has to export the imagery data from the image service in both cases?

Answer 1

From my testing so far, it seems like this happens because exporting the map document to PDF does not necessarily keep the image at as high of a resolution as exporting the image service image to a raster file. In particular, with data frame extent being equal, the DPI of the PDF seems to be the biggest factor. I hadn't realized this before but at 300 dpi, the image on the PDF is at a noticeably lower quality than the image service on ArcMap when you zoom in close. Increasing DPI from 300 to 600 made the PDF export take 3-4 times longer and the image looks to be about as high quality as ArcMap's.

I think variable server traffic could have also been playing a role at the time I was writing my question and causing those particular raster exports to take much longer than usual.

For the curious, I'll also say that increasing pixel depth on the raster export (from 8-16) doesn't affect export time very much, but it DID cause the exported TIFF to be totally black upon getting added back in to ArcMap. To fix that I had to go to that layer's Properties > Symbology and check "Apply Gamma Stretch" and it was back to normal. And increasing cell size lowers export time as well, given that you are effectively lowering the image resolution.