Question for GIS people

[nvprospector]

How's this? I georeferenced with county and state boundaries. As you can see, the points don't really match up. I've georeferenced a lot of old maps and that's just the way it goes. I didn't do 2nd or 3rd degree polynomial transformations (what craig and DaveM call rubber sheeting I think) because there was too much distortion.

I can send you the JPG with the georeferencing file, assuming Ozi can use that data

Ozi can read georef files. Looking at what you have laid out is the same results I ended up getting when bringing in the map to Ozi. Importing the map is not the issue. The issue is I need to create some sort of datum in order to use the map with moving maps and a GPS system and I am aware that it won’t be accurate.

Ozi only has the currently used datum’s for the globe and the oldest datum that is listed is NAD27. One is able to add different datum shift into the program and this is what I am working on. This isn’t the first time I have wrestled with an old map, it is for some reason this map is giving me trouble and I can’t figure out a datum shift that will work for the map and it is driving me crazy. Maybe I need to step back and take a different look at the issue.

Thanks for the advice thus far.

-Tim

 

[nvprospector]

That's what I was suggesting too, with one caveat.

The datum will become a problem if the lat/lon lines on the original map are used because those lines are tied to a datum. If map features other than the lat/lon grid are located on the original map are identified on a second map where the datum is known, he can rubber sheet to the datum/projection of the known map.

Craig

I will give this a try. I have a map that is from the 1910 census for the state that will work for this attempt. It is roughly the same size and I do have the projections and the datum shift programed into Ozi already. It will be very easy to move things around to make it work

Tim

 

[winkosmosis]

What do you mean by datum shift?

Why can't you use whatever datum is default?

 

[nvprospector]

I can send you the JPG with the georeferencing file, assuming Ozi can use that data

I would like to see that georef file if you don't mind. I would like to see how far I am off with your projections. I am still new with GIS and I still like to compare results.

-Tim

 

[nvprospector]

What do you mean by datum shift?

Why can't you use whatever datum is default?

Geographic coordinates based on a different datum (and spheroid/ellipsoid) then the one used to obtain the original coordinates. Or, when one converts NAD 27 to WGS 84.

 

[winkosmosis]

PM me your email address and I'll send the JPG and all the files that go with it (the original TIFF won't work with the georeferencing files).
I'm having weird issues though... It now shows up in Arcmap as an all black image. I'll try viewing it again this weekend.

A datum is really a model of the surface of the earth (sphere, ovoid, lumpy ovoid, etc), and with such an old imprecise map it doesn't have any relevance, especially after georeferencing. If you can, set OziExplorer to the state plane for Nevada, and georeference to that.

 

[nvprospector]

A datum is really a model of the surface of the earth (sphere, ovoid, lumpy ovoid, etc), and with such an old imprecise map it doesn't have any relevance, especially after georeferencing.

This makes sense on the issue being an imprecise map.

If you can, set OziExplorer to the state plane for Nevada, and georeference to that.

This I can do and will give it a try.

Thanks

 

[winkosmosis]

BTW, this is where I got the base data

http://www.blm.gov/nv/st/en/prog/more_programs/geographic_sciences/gis/geospatial_data.html

 

[nvprospector]

BTW, this is where I got the base data

http://www.blm.gov/nv/st/en/prog/more_programs/geographic_sciences/gis/geospatial_data.html

Nice site. Thanks.

 

[DaveM]

What do you mean by datum shift?

Why can't you use whatever datum is default?

Ditto this question. Once you register or rubber sheet the image it will be whatever datum and projection you reference it to. You are assigning a datum and projection. The image is warped by means of control points to match an existing, known datum and proj (what ever you pick). A datum shift is different, where you are updating the existing projection of a map to a newer datum. This involves displacement of the original co-ordinates on the map to (hopefully) more precise locations but is not a transformation by altering the map image shape itself.

Once your map is registered to a projection you don't need to worry about any datum shifts or what it's original datum was.

 

[craig]

I agree 100% with David's last response.