Cropping an ASCII DEM...

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alarduran
Posts: 51
Joined: April 17th, 2020, 6:03 pm

Re: Cropping an ASCII DEM...

Post by alarduran »

Hi, Wojo:

I checked the coordinates and they are correct. However, there might be a problem with the DEM-file units, as the software I use to crop it says they are meters, not yards or feet, as I was expecting. I think I have to check the units of the cropped DEM-file, as I suspect they might not be the units that APCD expects. I am looking into it at this moment.

Regards.
alarduran
Posts: 51
Joined: April 17th, 2020, 6:03 pm

Re: Cropping an ASCII DEM...

Post by alarduran »

Hi, Wojo:

Ok, this is what I found: when cropping the DEM file (30x30 meters per pixel) to the desired coordinates, I obtained a rectangle that measures 1,447 meters horizontally and 2,018 meters vertically (that is: about 1,582 x 2,206 yards). Then I save it as an USGS DEM (in feet) and imported it into APCD. Then I got a terrain measuring 4,626 x 6,496 feet (that is: about 1,542 x 2,165 yards). So I am losing about 40 yards in some step. I attach you the cropped DEM file and the resulting crz file as well.

Regards.
CroppedDEM.zip
(144.04 KiB) Downloaded 135 times
alarduran
Posts: 51
Joined: April 17th, 2020, 6:03 pm

Re: Cropping an ASCII DEM...

Post by alarduran »

More findings:

I was able to determine the UTM coordinates of your yellow box (X from 418013.288 to 419460.147 meters, Y from 4969642.403 to 4971659.939 meters, that is, a 1,447x2,018 meters area). I made a cropped DEM with the original 30x30 resolution and 100 meters wider than the UTM coordinates.
Then I tried my best to import a terrain with the yellow box coordinates into APCD, selecting a 1,448x2,014 meters area.
Image1.jpg
Image1.jpg (117.84 KiB) Viewed 2435 times
Well, after click import, I got a 4,920x6,691 feet area (that is, a 1,500x2,039 meters area).
Image2.jpg
Image2.jpg (252.85 KiB) Viewed 2435 times
So APCD added about 50 meters horizontally and about 25 meters vertically to my selected area... by his own. Why? I guess because he makes 30x30 meters cells (98,40x98,40 feet cells) and covers the area you selected with an whole number of cells, extending your area if he needs to.
Regards.
wojo
Posts: 66
Joined: August 2nd, 2020, 4:17 pm

Re: Cropping an ASCII DEM...

Post by wojo »

OK... now I'm officially confused. I just posted a topic "DEM Point vs. DEM Gradient Images". I've learned that the old USGS DEMs might be pretty inaccurate to begin with. I'm wondering whether APCD uses the XYZ points listed in a DEM file or does it use the Greys in a gradient image. If I ever get it sorted out, I'll post the answer here.

Wojo
wojo
Posts: 66
Joined: August 2nd, 2020, 4:17 pm

Re: Cropping an ASCII DEM...

Post by wojo »

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
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