Problems Mosaicking LiDAR DEMs

By GIS_at_TNC Date 2013-02-13 21:03

I'm trying to use ArcGIS (v. 10.0) to mosaic about 30 LiDAR DEM rasters into one new raster.
Each time, the output raster contains just one raster extent, the corner raster that represents
the 'top of the list' of rasters that need to get mosaicked.

I've tried the following ArcToolbox tools, each without success:
- APPEND;
- MOSAIC to NEW RASTER
- RASTER CALCULATOR
[ using this expression: Con(IsNull("LiDAR_DEM_Minnesota\dem01"),0,"LiDAR_DEM_Minnesota\dem01") + Con(IsNull("LiDAR_DEM_Minnesota\dem01"),0,"LiDAR_DEM_Minnesota\dem01") ]
- CREATE MOSAIC DATASET, followed by ADD RASTERS TO MOSAIC DATASET.

All input DEMs have defined coordinate systems (UTM Zone 15, NAD83), are stored in individual FGDBs, 1m pixel, 32bit float.

Output goes to a different FGDB.

Any suggestions as to what I'm missing?

Thanks,
Jan S.

By SteveKloiber Date 2013-02-14 21:01

Jan:

Have you checked your environment settings in ArcGIS? This sounds like maybe your processing extent is defaulting to the first raster file in the process rather than the union of all raster files.

Steve Kloiber
DNR, MN.IT

By GIS_at_TNC Date 2013-02-14 22:06

Yes, I've set the Env. Var. Processing Extent to UNION OF INPUTS.
This doesn't appear to fix the issue, regardless of the tool that I test.

All input rasters are named "dem01", but I can't imagine that would
cause this issue?

Thanks for your suggestion!

Jan

By SteveKloiber Date 2013-02-15 18:40

Jan:

I'm not sure what is happening. So you set your output extent parameters. Have you checked the actual extent of the output file? Is the process creating a new raster file with the proper extent, but simply filling in the area outside the first DEM will a "No Data" value?

Steve

By ? Date 2013-02-15 21:12

Steve,

I've compared the extent values of the input rasters with
that of the supposed mosaicked output raster,
and again it indicated failure.

So, there's no expanse of NoData in adjacent areas where
there should be mosaicked raster data.

Stumped...

Jan

By SteveKloiber Date 2013-02-21 19:56

Can you get it to work for even just two tiles?

By Tim Loesch Date 2013-02-15 18:20

Jan,

I have not run into this before and I have mosaiced literally thousands of these - have you resolved the issue?

tim

By ? Date 2013-02-15 21:05

No, I've not resolved this issue.
If you're interested, the LiDAR Data are for Dodge County, MN,
FGDB names are:

5142-02-31 through 5142-02-39
5142-03-31 through 5142-03-39
5142-04-31 through 5142-04-39
5142-05-31 through 5142-05-39

So, 36 rasters in total.

Any suggestions are much appreciated!

Jan

By Tim Loesch Date 2013-02-19 17:55

Jan,

I just downloaded all 36 tiles and they mosaiced together quite nicely without any trouble.

Download it from: ftp.dnr.state.mn.us/pub/outgoing/dodge_elevation.zip

Can you contact me directly at tim.loesch@state.mn.us? I have something I would like you to test.

tim

By ? Date 2013-02-19 16:03

What did you use to mosaic your LiDAR DEM tiles, Tim?

Thanks,
Jan

By Tim Loesch Date 2013-02-19 21:22

I used ArcGIS V10.1 sp2 but I have been doing this for a long time using v9.3, 10, and 10.1.

I normally create the FGDB, then a Raster dataset (32 bit floating point) and then use Mosaic_management and then create pyramids.

When you mosaic make sure that you reference the full path to the tiled rasters - if you are adding the DEM01 rasters to the data frame and then adding those rasters to the Mosaic tool, it may be confusing the names as they are all DEM01.

tim

By GIS_at_TNC Date 2013-03-08 21:32

Lessons Learned:

With the help of Tim and Steve, as well as colleagues at the office,
we've determine the following best practices:

- Create a new File Geodatabase to receive the Mosaic to New Raster Output raster(s).
- Set your Environment Variables carefully: especially the Processing Extent to "Union of all Inputs"
- Make sure you rename the input DEMs to unique file names (e.g. not dem01, but dem01_5142_02_39)
- Don't load up more than 65 Rasters into a single Mosaic to New Raster tool, that seems to be a limit.

These findings were based on ArcGIS 10.0 SP 5 running on Windows 7 Pro, 64bit.

Thanks to everyone for helping to trouble-shoot this issue.

Jan

By Tim Loesch Date 2013-03-11 15:24

A couple of notes pertaining to the lessons learned.

1) Good advice!

2) In addition, make sure you set the snap raster to be the first one in the list so that the origin is properly registered.

3) When Jan refers to making sure you rename the input DEMs to unique file names - this ONLY applies to the LAYER name in the Table of Contents - Not to the raster itself. ArcMAP Tools cannot differentiate among layers in the Table of Contents that have the same name so it picks the first one in the list.

4) I regularly mosaic hundreds of rasters at the same time so 65 is not a limit. Just last week I mosaiced 693 individual DEMs in a single pass for the Central Lakes Block 3 acquisition.

tim