Base Layer Alignment - Part 2

Jimmie's Avatar

Jimmie

26 Oct, 2020 06:11 PM

I am going to try this again because the topic was closed for some reason, but there was no clear answer given. In addition, I am a PAID Pro User.

Here is my original inquiry:
https://help.caltopo.com/discussions/bugs/5145-base-layer-alignment

I read the information in the help link provided but it does not help me in fixing my map. How do I fix this alignment issue? Is there a way to simply drag my copied trail left or right and re-align it on the base map? The water feature on my map is a key location and not having it line up correctly is not an option. How do i correct this?

Thanks,

  1. 1 Posted by Jimmie on 26 Oct, 2020 06:12 PM

    Jimmie's Avatar

    The maps again if you would like to re-examine.

  2. Support Staff 2 Posted by Julie on 26 Oct, 2020 07:32 PM

    Julie's Avatar

    Jimmie,

    You can re-open a discussion at any time by replying to it again.

    I assume that you took a GPS track of your trail in order to draw that on the map. Unfortunately, give the terrain (with the bluff to the immediate west) and tree cover in that location, it is much more likely that your track is off by ~50 feet to the east, and it just happens to match better with the less accurate scanned topo layer than with the more current USGS hydro data used in created the MapBuilder Topo, TF Outdoors, Google maps, Open Street Map, and more. We have an additional article about GPS accuracy here.

    There is no alignment issue with the base layers; the maps are drawn differently. Specifically, the scanned topo layer is literally that- scanned USGS topos. We didn't scan them or georeference them. The USGS did that. Additionally, as referenced in our article, these USGS quad maps were created with some amount of error expected. "...the USGS only guarantees 90% of well-defined points to be accurate to within .02 inches of their actual positions on maps with a scale smaller than 1:20,000." This translates to around 33 feet at that scale, and that's only guaranteed for 90% of major points. The difference between the stream location in the screen you reference is about 50ft. Furthermore, the maps that make up the scanned topo layer were made decades ago, and it is very possible that the size of the pond feature has changed over the years, or that modern cartography techniques have created a more accurate map. The scanned layer is provided as a reference, and because some users are more comfortable using the familiar look of the old paper topo maps from the USGS.

    Again, this is why we provide many different base layers and overlays. Maps are not perfect. You will find small variations in the location of features across all types and sources of maps, and an even larger variation in GPS tracks.

    Best,
    Julie

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