Author Topic: Optimal DEM file size (Read 4179 times)

Metzen · « **on:** February 20, 2007, 11:44:32 AM »

I am downloading DEM data for the entire state of Utah. It's on the order of 1.2 GB of data.

Is there an optimial file size to break the data into? You can download it in 250 MB, 100 MB, 75 MB, etc.

Will the 250 MB DEM files slow down the 3D viewing in TF?

Thanks!

Metzen · « **Reply #1 on:** February 20, 2007, 11:46:16 AM »

As a quick follow-up, I'm going to be downloading other surrounding states as well.

If you get to where you have 3 or 4 GB of DEM data, is that going to slow down TF startup and usage? How and when is that read?

Thanks again!

Krein · « **Reply #2 on:** February 21, 2007, 10:58:34 AM »

Sorry about the delay in response. I had to get some info on how it works since I didn't write the DEM reading.

On startup the program reads the demIndex.dat file which contains all the header info for the dems. That file should be too large. It also checks for new/deleted DEM files. So startup time shouldn't be bad.

The indexing part of TF is pretty fast, so having many DEM files shouldn't be much of a problem. But the deal killer is actually loading the DEM file.

DEMs are only loaded as needed, but if you need any piece of a particular DEM file you must load the entire thing. So in this respect the smaller the DEMS the better.

I've got all of AZ as quad-sized DEMs, but I rarely use it. I usually just use the 3D download. If I'm doing views of large areas it's often faster. But it is nice to have the DEMs for offline use and when the elevation server is down.

Metzen · « **Reply #3 on:** February 21, 2007, 11:06:37 AM »

Thanks for the response. That is pretty much what I came up with this morning, and was downloading the DEM's again in 25 MB chunks when you replied.

I appreciate the response.