How to georeference a scanned map

Earlier we talked about how to do this procedure using Microstation, and although it was a lowered image of Google Earth, it is applied equal to a map with defined UTM coordinates.

Now let's see how to do the same procedure using manifold.

1. Obtaining Control Point Coordinates

It requires at least four map points with known coordinates ... eye, and with a projection also that could well be NAD27, WGS84 or other. Generally these coordinates are in the corners like the image below.

In the case of a map that does not have coordinates, these can be taken in the field with a GPS, using clearly identifiable points. In this case I used a Thales Mobile Mapper base and another to capture the points after differential correction have done with Mobile Mapper Office and the accuracy of the points is submeter.

Once the data is obtained we store it in an excel file, with two columns, one for coordinates x (lengths) and one for coordinates and (latitudes)

To import the image to Manifold from usa file / import / drawing and select the scanned image.
2. Import Control Points to Manifold System

From Manifold we select file / import / drawing and select the xls file option, then find the file.
A dialog box appears, you must select the columns that contain the data of interest.

Columns: keep the check in the columns X 'and Y', in the others remove the check.

X / Longitude: X '

Y / Latitude: Y'

Now press Open, and the table is automatically imported into the manifest file.

3. Assign projection to imported data

In the Project window we right-click on the newly imported drawing (ends with *Points drawing) and in the displayed menu select Assign Projection. Configure the Coordinate System in this case we will use UTM zone 16N, and datum WGS84.

4. Creating control points.

Access the menu "Tools / Customize" and a dialog box is displayed where we select the box corresponding to Control Points and then Close.

On the right side of the screen the Control Points, click on the tool New Control Point, and we located each control point with the help of Snap to Points, this in the case of the map whose corners we knew.

In the case of the map from which we have taken points with the GPS, we import the DXF file with the option File / import / drawing and we select the category dxf files. The rest is similar to the previous step, always assigning projection with the steps of point 3.

5. Georeferencing the scanned image

To import the selected image file / import / drawing by choosing the option jpg files, or format having our image. Then we add projection following the steps of point 3.
Now open the image by double clicking on it, and select the tool New Control Point, marking each point in the same location as the points indicated above, and exactly in the same order in which they were assigned on the map.

Now in the Control Points box we select Sign up, y in the displayed box we check the following parameters:

• Reference: Mosaic
• Method: Affine (scale, shift, rotate) then press OK
  
  To save the image in a georeferenced format, you just have to right-click on it, and then export. You can save .ecw format that is fairly light.

Of course, the best way to georeference a google image Earth (or Virtual Earth, or Yahoo maps) with manifold is stick directly to image service ... and you save all this battle.