Creating a map: highest resolution?

[jwfrankman]

Hey all,

Thanks for this site:  you all are really good at what you do!

After not being able to figure out how to make the "MN Lake Depths" work that's posted on this site (apparently no one can figure it out?) I decided to follow the instructions for using Google Earth to make my own.  I downloaded a jpeg image of a lake (with contours) from the MN DNR website and all went well...it appeared in basemap and I was able to view all areas, mark waypoints, zoom to see individual contours, and it loaded perfectly into my 62sc.

However, when I attempted to zoom in on my unit in order to navigate to a certain depth, I discovered that the resolution leaves something to be desired.  When making the map, some trial and error showed me that the size of the image can't exceed something like 3mb (?), and I pushed it right up to that limit.  Is there some method of increasing resolution on a map like this that would allow me to zoom and be able to at least somewhat see lake depth contours?

Thanks in advance,

Jason

p.s.  I'm a GPS noob, so be nice   

[Boyd]

I'm not sure if there is a limit to the resolution that can be used - I made a map at 4 inches per pixel myself: http://forums.gpsfiledepot.com/index.php/topic,1335.msg8387.html#msg8387

But I think you have a different issue. Each image can be no larger than 1024x1024 pixels. I think you are also right about the 3mb size, but that should not be an issue. What is the source of the image you're using and what is the resolution?

You can have multiple images in the same map, up to a total of 100. So you need to do a little simple math to understand the limitations. If each pixel represents 1 foot, then each individual image ("tile") would be 1024x1024 feet. If you have 100 of these arranged as 10 x 10, the map would cover 10,240 feet x 10,240 feet - about 2 miles x 2 miles.

To make this kind of map, you will want a better tool than Google Earth. Have a look at g-raster: http://moagu.com/?page_id=155 The author is a member here and often answers questions for users.

Another helpful tool is mapc2mapc, which helps to create irregularly shaped maps so that you can maximize the use of the 100 tile limit: http://www.the-thorns.org.uk/mapping/

[jwfrankman]

Thanks for the reply!

The lakemap in question is available at the MN DNR website as either a pdf or tif file, and is about 140k or so.  I also have a scannable copy of the map, which I haven't done yet but still may.  Maybe I'd get better resolution if scanned it, but then I suppose you're at the mercy of your scanner's performance?  If I scanned it at the highest resolution possible, would that help my cause?

I'll give g-Raster a try--looks easy to use, and I wouldn't mind paying for the registered version.  I read through the limitations of the freeware version: do you think I'd be able to get a pretty good idea of its functionality before buying?  I saw the 1500x1500 pixel limits on kmz files and 2000x2000 pixel limits on gpdf to gtiff conversion and wondered if I'd be able to at least clearly see what it does before purchase...

Jason

[Boyd]

Sorry - I have not used either of those programs very much myself, I use GLobalmapper, which is a somewhat expensive GIS program. I can tell you however, that each tile in the .kmz file must be no larger than 1024x1024 pixels. It will still work if you use larger tiles, but the rendering quality will be degraded. That might be the problem you're seeing.

You need to have a handle on the resolution of the map you're trying to scan or download. For a printed map, 10 feet per pixel can usually show everything. If you scan a paper map, scan the scale also, then use Photoshop or another image editor to figure out how many feet each pixel represents.

I believe g-raster can automatically open geoPDF and geoTIFF files, so if you can download those then the program will probably figure everything out for you.

[leszekp]

G-Raster author here. With the unregistered version, you'll be able to see immediately whether the program meets your needs. With KMZ files, it will show the center 1500 x 1500 pixels in full resolution, same with the upper-left 2000 x 2000 pixels for GeoPDFs. Just tested it on a PDF downloaded from MN DNR (Wellner-Hageman Lake in Brown County), and it worked fine. You'll need to run the GeoPDF tool first to convert it to GeoTiff format, then take the GeoTiff and load it into G-Raster.

If the TIFF from DNR is a GeoTiff (meaning it has calibration data embedded in it), the unregistered version will process the full image, no size limit.

[jwfrankman]

Good info, thanks!

I had a lake map that I scanned into a .pdf at three resolutions, 300, 350 and 400 dpi, which corresponds to about 11, 10 and 8.5 feet per pixel.  The file sizes were 860, 940 and 1140 k, respectively.

Do the raw file sizes matter with your program and/or installing them on the gpsr once they're chopped up and spit out?  Sorry if I'm asking stupid questions but this is all pretty new to me.  I haven't had time to download g-raster yet, but I plan on doing it these next couple of days...hopefully I'll get a better idea of how to do this stuff.

Thanks,

Jason

[leszekp]

A few thoughts:

- You don't want to scan the map at a resolution higher than is absolutely necessary, as this increased the overall size of the map. However, G-Raster has a "Rescale" function that will shrink an image down to a lower resolution to reduce the space required. My guess is that 200 dpi should be more than sufficient for these maps.

- If you can get the PDFs directly from MN DNR, that's a better choice, as they appear to be GeoPDFs that are already geographically calibrated. For your scans, you'll have to manually calibrate them either in Google Earth, or by using a "georeferencing" program. For both of those, the image will have to in a standard graphic format (e.g. JPG, TIFF), and not PDF.

- The largest image size the program can handle is 100 megapixels, which is convenient because that's about the limit the number of image tiles in Custom Maps for most Garmin units, 100 one-megapixel tiles (Montana units can handle 500 tiles). If your image is too large, there's a Chop/Crop/Shrink tool that can massage the image into a more compliant format.

- Read the help file; I've spent a lot of time on putting in as much info there as I could.