Lake Bathymetry Sources / Offline Map File Conversion – September 7, 2022 at 09:40PM

This may be a little outside the norm but I’m at my wits end with this and hopefully if anyone can help it will be from this group….

I’ve been tasked with making a small model of a local lake (Lake Townsend, Greensboro NC, USA) for the purpose of display/education. I have a CNC router and have taken on similar projects, but this one keeps hitting a series of dead-ends. I’ve gone down some very deep and intricate tool chains trying to solve this, but to summarize/simplify: (With questions for each path in parenthesis)

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#1 – Exhaustive search for official data. The usual suspects (GIS, USGS, NOAA, Army Corps) don’t cover bathymetry for this area. Through an information request to the city I was able to get a higher-res raster PDF than was publicly available, but nothing in any vector format. I experimented with a few vector tracing tools, but realized even with automation this could potentially take weeks of manual tracing and cleanup; a non-starter. (Are there other sources that I’ve overlooked?)

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#2 – At least two commercial sources have vector depth data (gpsnauticalcharts.com, navionics) for this lake, but neither are geared towards GIS use. I took a chance and bought the ability to download offline maps for the 1st, thinking I may be able to use the offline file. The file appears to be MBtile format; I can open it as a sqlite database and see X/Y/Z/tiledata columns, but it lacks header information and I haven’t been able to find a filetype (i.e. svgz) that works with the data provided in each tile. QGIS seems to have the same problem – it successfully parses the file as a database but it can’t make sense of the tiledata. (Has anyone had experience with these offline data files? or does anyone know of another service that produces offline files that can be used in this way?)

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Once I can get just about any kind of clean vector format with only contiguous loops at each elevation (like any topo map) it’ll be easy to run through to CNC as usual, but having trouble getting there….