Global Mapper v25.0

creating contour map/lines

amalayer
amalayer Global Mapper User
edited November 2014 in How I use Global Mapper
how can i create a contour map without dem data (so i cant just click generate contours)... all i have is a topo map.. (see attached)
contour_sample.jpg

i can download it from an online imagery but i want to learn how to create contours... it looks complicated but im interested how..can someone please teach.. thanks a lot..

Comments

  • yianni
    yianni Global Mapper User Trusted User
    edited August 2015
    mmm I don't think you can. As GM is concerned, these lines are just an image and they do not have any elevation info in them. It could just as well be the picture of a girl and you asking GM to create some contours...

    You can draft lines over them, if you have the patience and export these as a contour file in another format.

    Or download ASTER/SRTM data and create the contours of them.

    or maybe wright a script that will read the color intensity (green vs brown) and assign lines where it sees a brown pixel?
  • PhilRatcliffe
    PhilRatcliffe Global Mapper User Trusted User
    Great answer yianni, I have never thought of the colour intensity option. Good stuff
  • yianni
    yianni Global Mapper User Trusted User
    :smile:

    glad you like it. I am not very good at writing scripts. started having a go but haven''t devoted enough time on this yet.

    if it works it will be really good i think. I wonder if they can implement it on GM as well as a function. I guess it will create points along the contours and then it is ever so easy to create a line out of them.

    same thing could be used to define any feature of a map come to think of it. lakes, land mass etc. Though the less color variations the better it will be. i.e. it will be easier on an admiralty chart than a sat. image.

    i am sure there is more to it and it might not be very straightforward but hey ho. could be an interesting challenge.

    y
  • PhilRatcliffe
    PhilRatcliffe Global Mapper User Trusted User
    I would certainly like to know, afraid complex script writing is beyond me though.
  • Ice Age Mark
    Ice Age Mark Global Mapper User Trusted User


    Gentlemen,

    My experience with GM has been that the only practical (though tedious) way to get digital contours from a raster image of a topographic map is to manually digitize (trace) them. As Yianni stated so well, it's really only a picture of a map, even if it's georeferenced. The problems associated with getting a final product by image extraction are just not worth the effort compared to digitizing. At the very least, when you zoom way in, you will see that the contour lines themselves are not discreet brown pixels, but instead, they transition into the color surrounding them. Even with scripting, I would not recommend using time on trying to create an extraction process. If you really must have them from that source, just get busy tracing. GM has some functions that really help with that. Don't even waste time filtering or adjusting the image, the contours are easiest to see and follow on the map in it's original imaged form, that's why it was produced that way.

    Best of luck,

    Mark
  • PhilRatcliffe
    PhilRatcliffe Global Mapper User Trusted User
    Thanks Mark, just saved me a lot of trial & error time. Cheers Phil