Why 100K in Tutorials?

[phuber]

Some background: I'm a cyclist and I've been a computer systems admin for 25+ years. About six years ago I hacked together a map of the Angeles National Forest and loaded it on my Etrex which I then took MTB'ing into the wild. The methodology for creating the map was striking similar to that in the Tutorials section of this site. There were two main differences:

1) My main motivation was that I wanted 24K rather than 100K topo maps. In the tutorials now it seems like the user is told to use 100K. Period. Is there now some limitation on the .gov stuff which didn't exist in the past?

2) At the end of the process I just popped out the memory card and mounted as a disk on a linux box and copied the IMG file I had created into the top level of the card's file structure, naming it "gmapsupp.img". Plugged it in and voila! Does anybody know if that'd fly on a nuvi? -pat

[Seldom]

1) My main motivation was that I wanted 24K rather than 100K topo maps. In the tutorials now it seems like the user is told to use 100K. Period. Is there now some limitation on the .gov stuff which didn't exist in the past?

No limitations I am aware of on USGS data regarding elevation grid size except download time you are willing to tolerate.  I usually generate contours using 10 meter grid data.  If you were reading the Hydrography section, 100k refers to something else.

2) At the end of the process I just popped out the memory card and mounted as a disk on a linux box and copied the IMG file I had created into the top level of the card's file structure, naming it "gmapsupp.img". Plugged it in and voila! Does anybody know if that'd fly on a nuvi? -pat

It works on my old nuvi 700.  It was a long time ago, and the map was transparent topo to overlay on CNNANT.  IIRC a prompt asked if I wanted to add my gmapsupp to the internal data.  I said no.

[phuber]

Got it, thanks. I'll be getting the nuvi in a few weeks and report how it goes. BTW, things have come a long way, looks like I'll be able to DL some cool pre-made maps from this site.

[Seldom]

Trail data is always at a premium.  If you get anything new you want to share be sure to let jbensman know.  He maintains My Trails.  Also, OpenStreetMap is a good place to upload trail data.  If you put it there correctly, the whole world has access to it, and it's routable.

[Indrid Cold]

similar to that in the Tutorials section of this site. There were two main differences:

1) My main motivation was that I wanted 24K rather than 100K topo maps. In the tutorials now it seems like the user is told to use 100K. Period. Is there now some limitation on the .gov stuff which didn't exist in the past?

the 100K mentioned in the tutorial is for the map sizes, not the resolution.

[Boyd]

Right - I was just looking at that again myself. The map is sliced into individual tiles, and the boundaries of those tiles match the boundaries of USGS 100k maps. I also noticed the article that explains this is no longer properly linked, soI  mentioned this in the site support forum.

FWIW, on the Map of NJ that I just released, I did use 24k quad boundaries for the map tiles. This results in a lot of tiles (~300 for New Jersey which is only about 9,000 sq miles). However, due to the high amount of detail in my map I was having performance issues on the GPS when using bigger tiles (previous version of the same map only had 35 tiles).

So if you are creating a map with lots of detail (such as landcover), you can improve performance by keeping tiles small. I assume this is because a whole tile must be read by the GPS before it can display anything.

[maps4gps]

Great info Boyd.  There have been post asking about performance on other boards, but no one ever had any hard info.  I never tried to test it.  Now we have a delima - make larger quads/segments so all States can be on the GPSr or keep them smaller for better performance.  Anyone notice a difference with the large segments of built-in/pre-recorded-SD TOPO 100 versus the smaller segments used on the DVD?