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Up Topic Discussions / LiDAR Support / Where to find Wright Cty LIDAR 1m DEMS?
- - By chuck Date 2013-06-15 17:07
I downloaded the LIDAR readme files and was informed that 1m DEMs are located in the \data\county folder.  I have already downloaded Wright Cty LAZ files...converted to LAS...converted to TXT (data) files.
However...most Wright Cty LIDAR tiles (from LAZ/LAS/TXT) have an eastern boundary defect.  Most elevations near the eastern boundary are incorrectly increased by 1-8 meters! Merging two or more tiles thus produces a north-south
'wall' that is up to 8 meters high! Repairing (correcting) the incorrect elevations will entail weeks of individual data point replacement.

The 'readme' files state that 1m LIDAR DEMs are available in the \data\county folder.  However, the only such folder I could find for Wright Cty only contains the LAZ files!
I downloaded and unzipped the corresponding geodatabase 'gdb.zip' files.  For example: 3524-22-01.gdb.zip. Only GDBTABLE files such as a00000000a.gdtable are listed.

I also tried the "\data\raw\" suggestion but the website stated that such a folder does not exist.

I would appreciate receiving the correct URL address for Wright Cty LIDAR 1m DEMs.  I am hoping that the MNGEO generated DEMs do not have the elevation errors which are in the LAZ files.
Reply
Parent - By ? Date 2013-06-19 21:42
Chuck,
Sorry this has been so frustrating. This reply post is to answer your question about file locations. We are tracking down an answer to your second question about the “wall” you are finding.

First, make sure you are on the MnGeo FTP site ( ftp://ftp.lmic.state.mn.us/pub/data/elevation/lidar/ ), not the original DNR FTP site.

The folder structure has changed several times since the original readme files were written, so we will be updating the readmes very soon to refer to the current folder paths.  Wright County data is here:  ftp://ftp.lmic.state.mn.us/pub/data/elevation/lidar/county/wright/

The Situation Now
Currently, the 1-meter DEMs are only available in geodatabase format, for which you need Esri’s ArcGIS software. Their free viewer, ArcGIS Explorer Desktop, also can read that format, but it is intended as a viewer, not a fully functional GIS. Esri software uses all of the individual files, such as  “a00000000a.gdtable”, to show the user a 1-meter DEM feature class. That’s why you can’t find a separate 1-meter DEM file.

If you can use geodatabase format, you have two options:
1. Download the whole county file, named “elevation_data.gdb.zip”, which includes all the available layers (which are listed in this table: http://geoint.lmic.state.mn.us/lidar.php ).
2. Download smaller tiles in the “geodatabase” folder. The “tile_index_map.pdf” in the Wright County folder will show you the tile boundaries on a MnDOT highway map so you can figure out which tiles you need.

If you don’t have access to software that can read geodatabase format, Tim Loesch’s handout, “LAS File Processing Using LASTOOLS”  (ftp://ftp.lmic.state.mn.us/pub/data/elevation/lidar/LAS_File_Processing_Using_LASTOOLS.pdf ) may help in using the LAS data you have downloaded.

In the Near Future
DNR and MnGeo are nearly finished developing a new web browser application which will make the download process much easier and provide more options, including an open format (binary floating point) for the raster data. It will also provide an online viewer. The site will be in beta-testing stage next week and released soon after.  Keep an eye on MnGeo's LiDAR info webpage: http://www.mngeo.state.mn.us/chouse/elevation/lidar.html for an announcement of its availability, or let me know off-line if you'd like to be notified directly.

-Nancy Rader, MnGeo, nancy.rader@state.mn.us
Parent - - By Tim Loesch Date 2013-06-20 12:47
Greetings Chuck,

I will echo Nancy's comments about your frustration - hopefully we can point you in the right direction and look into the issues that you bring up about the data.

The 1 meter DEM that Nancy pointed you to was created from the LAS data that you downloaded.

It will help me a lot knowing a little more about the issue that you are seeing with elevations rising, improperly, to the east.

Is the issue widespread or did you see it only in a specific area?
What is the width of the affect area?
Is it right along the boundary of the county or is it on the very edge of the data?

A couple of coordinates would be very helpful.

tim
Parent - By ? Date 2013-06-20 14:06
:evil:I finally found out what my problem was...the "wall' effect was a 'gridding' artifact.  For some reason, it only showed up on the east side of each LIDAR tile. I tried quite a few gridding protocols and found that Kriging almost never generated the error, even tho' very computationally intensive. I have gridded uncounted elevation files, and never encountered this effect before. I wish to thank you for including the Lastools package. Converting LAZ->LAS->TXT is relatively easy when using a 64-bit OS and massive RAM and SSD.
Up Topic Discussions / LiDAR Support / Where to find Wright Cty LIDAR 1m DEMS?

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