Mosaic DEMs

[mpboyle]

We have received 2 DEMs for a project; one covers our entire county and has 6ft pixel resolution.  The other only covers a major city, has irregular boundaries, and is 3ft pixel resolution.  Since the 3ft DEM has irregular boundaries, there are areas of NoData within the DEM extents.

I have a few questions below since I don't work with rasters that often:

1. What is the workflow to mosaic these 2 DEMs together?
2. How do I have the 3ft DEM show on top of the 6ft DEM where applicable?...do I use ZOrder?  Do I set the ZOrder on the primary rasters or overviews?
3. Will having NoData values within the 3ft DEM extent cause an issue?...i.e., show as white space on top of the 6ft DEM?
4. Our 6ft DEM does not show at all extents for some reason.  When I am zoomed out to Full Extent, nothing is shown, why is that?

Any help would be greatly appreciated!

[DanPatterson_Retired]

You will have to resample the 6' dem to 3' or generalize the 3' to 6' if you want to have them together.

In either case, you can put them together by using the Con tool

[mpboyle]

Dan_Patterson‌,

Is this any different than having ortho-imagery that is 2 different pixel resolutions?  Shouldn't we be able to use a mosaic dataset to load in raster datasets (imagery or DEM or whatever) and be able to order the rasters to display?  I didn't think the varying pixel resolutions would matter with a mosaic dataset.

I suppose I understand if you plan on using the Con tool where the pixel resolutions would need to be the same since you're basically saying: if True, use this value; if False use this value...but for display/management I would think using a mosaic dataset should be a simple solution, no?

[DanPatterson_Retired]

Sorry Matthew, I didn't see mosaic data set as your desired output, I just saw mosaic which would require Con since two would become 1

[CodyBenkelman]

Matthew

I only have a moment to reply so will have to be brief - and might leave out some details - but this is doable (I originally wrote "relatively simple" but that's a matter of perspective  ).  

1. What you're seeking is the "Mosaic Method" which is the setting that controls which raster appears on top.  (See MD properties/Defaults tab) If you set Mosaic Method to "by attribute" and then choose "Low PS" (low pixel size), the higher resolution will appear on top.  
   1. You can also use ZOrder if desired, and in some cases we advise creating a new custom field (often called "Best") and then building an expression to combine something like date and cloud_cover to show most recent image *unless* cloud_cover is too great..
   2. If your users ever want to see the underlying DEM, they can use MosaicMethod = LockRaster which selects one or more rasters by ObjectID.
   3. Note for future reference that this setting places higher priority on lower numbers, so if you ever use this on a date field, you need to reverse the order to "Descending"
2. If you have issues with NoData values in higher res dataset hiding the underlying data, you can define one or more NoData value also in MD properties.  This works fine for a small number of rasters, but (for future reference) if you have a lot of overlapping areas, using the Footprint feature class is the preferred method of eliminating NoData.  (That can become a performance concern if you have a really irregular boundary e.g. at the edge of a lidar derived dataset - you don't want thousands of vertices in a footprint - in that case I recommend manually editing the footprint to just eliminate the data @ ragged edges)
3. Your overviews should not begin until you get to relatively low resolutio (Use DefineOverviews before BuildOverviews to control resolution, and read up on Pyramids vs. Overviews).  If you want to ensure the 3 foot data  is included in OVRs even at small scales it might take some extra work - let us know.  You said 6 foot data not showing when zoomed to full extent - are you saying the 3 foot data DOES show?  If so, that may be due to pyramids built on 3 foot data, pyramids missing on 6 foot, and no OVRs built.  If you Define then BuildOverviews, the data should appear @ small scale.

For MUCH more detail see http://esriurl.com/ImageManagement, follow link to the Image Management Guidebook and look for the chapter on Elevation.

I presume you've seen our raster functions for generating Hillshade, Slope, Aspect etc. on-the-fly from the MD?  Don't create separate rasters for those derived products unless there's a compelling reason...

Last note - most clients are okay with an obvious boundary at the edge of the 3 foot data, so users are aware if they're working at the edge of 2 datasets (perhaps with 2 different accuracy specs).  If you want to blend that edge so it is NOT obvious, that is doable, but definitely gets into more details...