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'm just going to focus on the problem. The following graph shows how between Costa Rica, Honduras and Nicaragua there is a change between the 16 and 17 zones; which implies that those coordinates marked in white circles are repeated. A point taken in the Honduran La Mosquitia, if it is not said to be in the 17 zone, would fall in Guatemala in the 16 zone, while the one in the Nicaraguan Atlantic coast would fall in the Pacific Ocean, just as it would happen with one in the Island of 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. In this way they will never be negative. But as a consequence, the coordinates are not unique, they are repeated in each zone 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 points, two will fall in Guatemala.
1 Convert UTM coordinates to geographical coordinates. This can be done with any program that goes 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 keep. The system will 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.
The only thing we do now is load the image. This is done from the raster manager, indicating that the image will be loaded interactively and indicate the top left point and then the bottom 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 geographical areas to have a single deployment. An ideal is in that area to lift points by configuring GPS to capture geographic coordinates.