Merging two datums to create single elevation dataset in ArcMap?

Wrangling Elevation Data: Merging Datums Like a Pro in ArcMap

Ever felt like you’re speaking different languages when trying to combine elevation datasets? That’s often because they’re referenced to different vertical datums – think of them as different starting points for measuring height. Ignore this, and you’re setting yourself up for some seriously wonky results. So, how do we get these datasets playing nice in ArcMap? Let’s dive in.

First things first, what’s a vertical datum anyway? Simply put, it’s the reference surface we use to measure elevations. There are generally two types:

• Gravity-related (Geoidal) Datums: These are based on mean sea level, which, believe it or not, isn’t the same everywhere! Gravity and other factors cause it to vary. NAVD88 is a common example in North America.
• Spheroidal (Ellipsoidal) Datums: These use a mathematically defined surface (a spheroid) to approximate the Earth. Your GPS, for instance, usually gives you heights relative to the WGS84 ellipsoid.

The problem? If one dataset uses mean sea level as its zero and another uses a spheroid, you’re comparing apples and oranges. That’s why we need to perform transformations – they’re the Rosetta Stone for translating between these different datums. I remember one project where we didn’t catch this early on, and the resulting flood maps were, shall we say, optimistic. Lesson learned!

Getting Ready to Rumble (or, Merge)

Alright, time to get practical. Before you even think about merging, you need to do a little detective work:

ArcMap to the Rescue: Vertical Datum Transformations

ArcMap has a handy trick up its sleeve: vertical datum transformations. This lets you convert elevation data from one datum to another, ensuring everyone’s on the same page before the merge.

Here’s the lowdown:

Merge Mania: Bringing It All Together

With all your datasets now speaking the same vertical datum language, it’s time to merge them into one glorious elevation dataset.

“Mosaic to New Raster” is Your Go-To:

Pro Tips for the Road

• Gold Copy is Key: Always keep a pristine, untouched copy of your original DEM data in its original coordinate system. You’ll thank yourself later.
• Tiled TIFF is Your Friend: For storage, Esri recommends the tiled, 32-bit floating-point TIFF format with LZW compression. It’s efficient and plays well with ArcMap.
• Overviews Matter: Think about how you’ll handle overviews (lower-resolution versions of your data). If you can snag some from other sources, that’s often the best way to go.
• Document, Document, Document: Don’t forget to document the vertical datum and transformation steps in your final dataset’s metadata. Future you (and anyone else using your data) will appreciate it.

The Takeaway

Merging elevation datasets with different vertical datums can feel like a headache, but with the right approach, it’s totally manageable. By understanding datums, using ArcMap’s transformation tools, and following these best practices, you can create seamless, accurate elevation datasets that’ll make your analyses sing. Now go forth and merge!