Friday, November 04, 2016

GIS "Pro" Tips - Merging Rasters

Often in GIS you come across situations where you need to merge together rasters in order to have one file to work off of (Digital Elevation Models (DEMs), aerial images, etc.). I have found that it gets a bit tricky sometimes to merge those together and get everything to align properly and/or not have any issues. Below is a list of suggested guidelines to follow while merging together multiple rasters. This was started and expanded upon from advice by a co-worker while I was working my GIS job and I figured I would share it since I could never find anything on the internet that was as clear and concise.

Tool: In the search field type in “Mosaic to New Raster”

                 In the Mosaic to New Raster Tool:

Input rasters: Bring them in any order, doesn't matter.
Output location: Save them to a different folder from the one they're in.  It just seems to help thing from crashing. Assign it to the C Drive.
Raster Dataset name with extension: Short name (13 characters or less), no spaces.  Do not give it an extension.
Spatial reference: Blank. It'll revert to whatever the projection of the source rasters is, which is what you want.
Pixel Type: 32-bit float
Cellsize: Blank.  It'll default to the sources' cell size.
Number of Bands: 1
Mosaic operator: Last…unless they don't overlap and/or your data comes from multiple sources.
Mosaic Colormap Mode: Default.

The important part:
Click on the Environments button then click on Processing Extent. Pick one of your input rasters as your Snap Raster. This makes sure that your new cell edges match up with the original and that nothing shifts over.  This is real important with datasets that have large cell sizes, but you want your data to line up regardless.

This is one of those tools that doesn't work sometimes for no good reason.  If that happens, just try again.

When merging 2 overlapping rasters of different tile size

Sometimes you may want to combine rasters with two different tile sizes. This may be you have a DEM and you want to incorporate areas with higher resolution data like LiDAR within the DEM area. To do this within the Mosaic to New Raster tool:

  1. Give the LIDAR DEM (or whatever the higher resolution data is) the mosaic operator highest priority.
  2. Set cell size as the same as the LiDAR DEM (or whatever the higher resolution data is).
  3. In environments:
    1. Set the raster analysis to min of inputs cell size.
    2. Use the LiDAR DEM as snap raster

Some useful information and websites

The USGS has a nationwide ~10 meter DEM map available for download for free.

The information is located at the National Elevation Dataset (NED):

You can find lots of free information available for various areas across the nation (DEMs, aerials, watersheds, etc.) and if you zoom into your area of interest you can find what information is available. The 10m DEM is referred to as the “1/3 arc-second DEM”. This means that it is approximately 1/3rd of a degree of latitude and longitude. The 1/3rd arc-second equals approximately a 10m DEM (A 10m DEM is a DEM where each pixel is 10 meters by 10 meters), while the 1/9th arc-second DEMs equal approximately a 3m DEM.

You can also find where freely available LiDAR data is located here:

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