I used a DEM file to create the mesh for the course I'm working on. At first I thought I didn't have my aerial photo lined up properly, because one of the fairways I traced was partially up the side of a hill in the mesh. Since then I spoke with a guy who knows a lot about DEMs and LAS files. He said that the old 7.5 minute DEMs could be very inaccurate vertically. I checked a bunch of my APCD grid points vs. the points you get by holding the hand cursor over the same spot on Google Earth. Way off in some places.
I can open a DEM file in Notepad and see what looks like a whole bunch of XYZ coordinates. Seems to me that if APCD is using XYZ points to create a mesh, that it should be pretty accurate. I've also seen those black through white gradient images. Does anybody know if APCD creates meshes from a point file or does it use that fuzzy image to set vertical points for the mesh?
I guess another question might be whether USGS used to store all that XYZ information in giant XYZ point files, or does it store the info in a photo? (That too might explain why the Z coordinates are all over the place. Maybe the XYZ points USGS gives you are bullshit that they interpret from the greys on a old stock photo?)
Anybody run into this?
Wojo
DEM Points vs. DEM Gradient Images...
Re: DEM Points vs. DEM Gradient Images...
I finally gave up trying to use the old DEMs. They are horribly inaccurate compared to LAS files.
I used a Lidar Widget called LAStoContourDXF from www.lidarwidgets.com. It can take only the ground points from the LAS to calculate contours at whatever interval you specify. It basically makes an AutoCAD DWG file that includes the contour lines and the LAS border... which matches the size of my APCD mesh. I used 20 foot contour lines. In AutoCAD I underlaid the course overhead image, which gave me image + contours. I screen captured that composite image and used it as the underlay to trace the course. I set my new flat course mesh to 10 feet above sea level because that's the elevation of my lowest fairway. Then it was very easy to capture the vertices on the high side of each contour line and drag them upward 20 feet until I reached 300 feet, the highest point on that particular plot of land.
It created a stepped APCD mesh that is pretty close when compared to the points you read off Google Earth. Then I captured enlarged Google Earth overhead images of each hole and used Google Earth to collect and record about a dozen or so points per hole... usually about 3 points along each fairway edge, 5 or 6 points around the perimeter of the greens, the tee boxes and the bottoms of the sand traps. As I go around the course fine-tuning the terrain, it seems no point on the APCD mesh is more than 10 feet off vertically. I also bought a GolfLogics Green Map to see the contours of the greens. I haven't gotten to that level of fine-tuning yet. I see it's going to take considerable time to get APCD greens that match the characteristics of the real ones.
Wojo
I used a Lidar Widget called LAStoContourDXF from www.lidarwidgets.com. It can take only the ground points from the LAS to calculate contours at whatever interval you specify. It basically makes an AutoCAD DWG file that includes the contour lines and the LAS border... which matches the size of my APCD mesh. I used 20 foot contour lines. In AutoCAD I underlaid the course overhead image, which gave me image + contours. I screen captured that composite image and used it as the underlay to trace the course. I set my new flat course mesh to 10 feet above sea level because that's the elevation of my lowest fairway. Then it was very easy to capture the vertices on the high side of each contour line and drag them upward 20 feet until I reached 300 feet, the highest point on that particular plot of land.
It created a stepped APCD mesh that is pretty close when compared to the points you read off Google Earth. Then I captured enlarged Google Earth overhead images of each hole and used Google Earth to collect and record about a dozen or so points per hole... usually about 3 points along each fairway edge, 5 or 6 points around the perimeter of the greens, the tee boxes and the bottoms of the sand traps. As I go around the course fine-tuning the terrain, it seems no point on the APCD mesh is more than 10 feet off vertically. I also bought a GolfLogics Green Map to see the contours of the greens. I haven't gotten to that level of fine-tuning yet. I see it's going to take considerable time to get APCD greens that match the characteristics of the real ones.
Wojo
Re: DEM Points vs. DEM Gradient Images...
this is why I like to take a week and make my own topo map, over lay onto a flat mesh and pull topo lines accross the course. DEM softwares can bring it to 1-2' intervals making it very detailed, depends on much time you want to spend. Its easy to overlay with a big 4096x4096 texture and accurate. cant stand messing with DEMs and lining them up, I can do it in almost the same time and I know it's accurate.