Scanned Maps

Question

I have scanned County Certification maps that have the Counties mapped out, a legend, and other info. All I want to do is trim out the County boundaries and save them for all the 69 Counties I have and georeference them together. I have since found out that I cannot do this because somehow the full scan is still there and when I georeference them together the other parts of the maps that I discarded still shows up. I have tried multiple editing programs and it always has the same result. Does anyone have any idea how to do this?

ChrisDonohue__GISP · Answer

What CBenkelman-esristaff suggested is the way to go.  Some helpful links if you are unfamiliar with Mosaic Datasets:

What is a mosaic dataset?—Help | ArcGIS for Desktop

Creating a mosaic dataset—Help | ArcGIS for Desktop

Chris Donohue, GISP

Cody Benkelman · Answer

Paul

I would recommend against editing a copy of the raster to save the copy to disk.  If you have (or can create) a polygon representing the county boundary, ArcGIS can use that to clip everything outside and consider it to be 'NoData' (transparent).

This is done by loading the images into a mosaic dataset (MD), then for each raster, loading the county boundary into the MD and defining the boundary polygon as the "footprint", then setting the MD property "Always clip rasters to footprints?" to Yes.

There's a slightly more elegant way to do this using "seamlines" rather than footprints to make it easier for a user to restore the map OUTSIDE the county polygon (e.g. if there is a legend or other info in the map collar they'd want to see).

If you've already done the editing, a faster (but not recommended) method may be to examine your edited tiffs and determine the pixel value outside the county pixels - probably either 0 or 255? - then you can define that pixel value as "NoData" and ArcGIS will make it transparent.  If all are loaded into a mosaic dataset, the NoData pixels should not obscure underlying maps.

[NOTE: I edited this post to add this comment]  One reason this is not recommended is: if your scanned maps have any of these NoData pixel values within the valid interior of the map, those pixels will also be transparent...  The "clip to footprints" method is usually better but I'm just trying to be sensitive to time you may have already invested in making copies, editing them, then georeferencing them.  But if you'll be doing more of this in the future, I'd strongly encourage you to follow our recommended workflow.  You'll avoid duplication of data, degradation of quality with resampling, AND the transparent "noData" problem

Cody B

PaulWirth · Answer

Chris,

Where I am having the problem is editing the non-georeferenced maps. I am taking 3rd party editing software to cut out the counties and save them as a tiff. I then georeference them and when I bring them into ARCGIS it somehow retains the original boundaries of the scanned certification map and blocks out the adjacent Counties that fit within the previously discarded and saved maps. In other words the original outline is preserved without me actually seeing it. I am stuck on this step before I even get to a workflow.

Let me know if you understand. I get the workflow. I just have to get there. I am stuck at editing the image.