We often have the problem of working in the limits of the UTM zone, and we see ourselves as sticks because the coordinates there do not work.
Because the problem
I explained some time ago How the UTM coordinates work, here I am just going to focus on the problem. The following graph shows how between Costa Rica, Honduras and Nicaragua there is a change between zones 16 and 17; which implies that those coordinates marked in white circles are repeated. A point taken in the Honduran Mosquitia, if it is not said that it is in zone 17, it would fall in Guatemala in zone 16, while the one on the Nicaraguan Atlantic coast would fall in the Pacific Ocean, the same would happen with one in the Isla del Caño in Costa Rica.
This is because the UTM grid takes a central meridian, with an x coordinate of 500,000, and from there it continues until reaching the zone limit. This way they will never be negative. But consequently, the coordinates are not unique, they are repeated in each area and in each hemisphere.
How to solve it
I'm going to use this example using Microstation Geographics now Bentley Map, it should be similar to AutoCAD: I want to georeference an image, having the four coordinates of its corners. In UTM it is impossible, because when entering the points, two will fall in Guatemala.
1 Convert UTM coordinates to geographical coordinates. This can be done with any program that is out there, before I presented a sheet Excel that does these times. As a result we will have this:
2 Change the coordinate system in Microstation. This is so that we can enter the points in that format.
It is made with: Tools> coordinate systems> master
Here we select the first icon (Edit master) and we indicate that the coordinate system is geographic. Always keeping the Datum WGS84.
Then we select from this panel the option Master and we save. The system is going to ask us some questions, to make sure we know what it implies, we accept all three times. From now on, we can enter the coordinates in latitude / longitude.
3. Enter the coordinates. This, for being few points is done via keyin; activating the command point, then from the keyin we write:
Xy = -85.1419,16.2190
We do the same for others:
- Xy = -83.0558,16.1965, enter
- Xy = -83.0786,14.2661, enter
- Xy = -85.1649,14.2885, enter
If you do not want to break the coconut you can save them in a txt and import them with the command that is Done for that.
Georeferencing the image.
The result of entering the points is there, on both sides of the boundary of the zone.
All we do now is load the image. This is done from the raster manager, indicating that the image is going to be loaded interactively and indicating the upper left point and then the lower right.
There they have it:
What about the plots:
Something similar would happen with the properties that are divided by the zone limit; what is done is that the vertices are converted to geographic locations to have a single display. An ideal is in that area to raise points by configuring the GPS to capture geographic coordinates.