Identifying the coordinate system or projection of a non-georeferenced map
In a previous blog, we discussed about how to georeference Adobe Illustrator document using MAPublisher. To recap, below are four of the required items to to make georeferencing possible:
- The scale of the map in the Adobe Illustrator document (e.g. 1 : 25,000,000, etc.)
- Details of the coordinate system the data is in (e.g. World Robinson projection, WGS84 geodetic system, WGS84 UTM Zone 32 N, etc.)
- The XY coordinates of one tie-in point in the coordinate system of your map in the Adobe Illustrator document
- The map's rotation angle.
We found that the most tricky information to collect is the coordinate system of the map. It can be very easy if it is clealy indicated in a map like this:
Often we found that many maps do not have this information included either on the map or in the form of metadata. One solution is to ask the map author about the coordinate system/projection of the map. If not, it will take a bit more sleuthing to make an educated guess about what coordinate system/projection the map is in. Before finding the coordinate system from the existing information available on a map, two assumptions need to be made:
Assumption 1: the map is drawn with a coordinate system or a projection assigned
If there is no coordinate system or projection with the map you are trying to georeference, there is no way to find the geospatial matrix of the coordinate system/ projection of the map.
Assumption 2: the map is scaled uniformly across the map
It is common practice that only selected parts of the map are scaled. In some cases, only the vertical or horizontal scale is adjusted. This approach is very popular when trying to make a map fit to a limited space or to make it more visually attractive. However, this technique may put you at a disadvantage because the map has been skewed and will be more difficult to georeference.
Suggestion on how to identify the coordinate system/projection of a map
Basically, we will collect the reference points from the map with XY coordinates (latitude/longitude), plot the points in a geodetic system (WGS84, using Google Earth), then compare positions of each of the reference points and each of the points plotted in WGS84. If necessary, transform (reproject) the point layer in WGS84 to an appropriate coordinate system, then again compare the position of the points to match with the collected reference points. If the position of each point matches, you have found the coordinate system of the map!
The example below is part of a nautical map, originally drawn with an earlier version of Adobe Illustrator without MAPublisher and, therefore, no georeference information is included
If you look at the original Adobe Illustrator document closely, there are graticule lines with latitude and longitude values availalble in the degree-minute format. We will use these graticule lines for georeferencing with MAPublisher.
Step 1: getting four reference points from the map
Get reference points on the map. Its suggested to have at least three reference points. If there are three points, we can detect whether or not the map is rotated with respect to the top of the document and the cartographic north.
We started with collecting the point at the top-left corner of the map by placing a point symbol (red symbol) at one intersection of graticule lines. This point will be placed in a layer named "my point".
Record the XY coordinates (latitude/longitude) values of each point. Below is the list of four points we collected on the map using the graticule lines. Note that longitude and latitude values are in the degree-minute format.
What would I do if there are no graticule lines?
It is common that there are no graticule lines included on a map. If this is the case, use the intersection of roads, corner of buildings, or some landmark point (e.g. lighthouse, bridge, etc.) Place the point symbol in one layer of your map (just like the one shown above), then record the XY (latitude/longitude) coordinate of the same position from Google Earth.
Step 2: create a GIS point layer in a geodetic coordinate system (WGS84) using Google Earth
Points plotted in Google Maps can be exported as KML or KMZ files. When a KML/KMZ file with plotted points is imported into Adobe Illustrator with MAPublisher, MAPublisher will create a MAP Point layer and a new MAP View. The MAP View contains the spatial information of the imported dataset and the coordinate system (in this case WGS84). Using this imported point layer as a guideline, we will start finding the coordinate system of the map.
Step 2a) Plot each of those collected points in Google Earth
Open the Google Earth preference options window. You can change the format of the latitude and longitude values. Tthe collected longitude and latitude values are in the degrees-minutes format, select the "Degrees, Decimal minutes" in the Google Earth preference option.
Start plotting a point. Click the point icon and enter the latitude/longitude values for one of the points.
Click the OK button to place the point to the specified coordinate values. Repeat this for the rest of the points.
Step 2b) Export plotted points as a KML/KMZ file
Once you complete plotting the collected points, you are now ready to export the points as a KML/KMZ file.
What would I do if there are no graticule lines?
In Step 1, we suggested to collected the points from known places such as the intersection of roads, buildings, or landmarks. Using Google Earth, please find the points of where you collected the points. If you place the point symbol by dragging, open the Property window for the placed point. You will see that the XY values (longitude and latitude) are indicated in the property window. Once you placed points at those collected spots, simply export the points as KML/KMZ file. The rest of the steps below will be the same.
Step 3: getting four reference points from the map
Now, we will import the KML/KMZ file created in Step 2. The imported KML/KMZ file will be in the geodetic system "WGS84". We will try to match the four points collected at Step 1 and the four points imported with the KML/KMZ file. If these points do not match, we will transform (reproject) the point layer with WGS84 to something else.
Take a look at this first example. The red points are the ones collected at Step 1 and the blue points are the ones imported from the KML/KMZ file. When comparing the position of those red and blue points, what do you think? Would you scale the point layer? Would you simply move the MAP View of the blue points? Would you rotate the MAP View of the blue points? or would you transform (reproject) the MAP View of the blue points to some other coordinate system?
How about this second example below? It looks like the MAP View of the blue points is rotated... but some other people might try to transform (reproject) the MAP View of the blue points to some other cooridinate system.
How about this third example below? We think it is a good example that those red and blue points match reasonably. In other words, you may have found the coordinate system of the map!
Step 3a) Import the created KML/KMZ file
Click the "Simple Import" button. Select the "KML/KMZ" file format, then browse for the file. As soon as you select a KML/KMZ file, MAPublisher will detect the coordinate system of the dataset and display it under "Source Coordinate System" in the Simple Import dialog box.
Once you click OK, MAPublisher will create a MAP View, which will contain all the spatial information including the scale and the coordinate system of the map (e.g. WGS84 is set as the source coordinate system of the MAP View for this example). The point layer imported from the KML/KMZ file is placed in the created MAP View.
Step 3b) Plot each of those collected points in Google Earth
For demonstration purposes, we changed the point symbol for those imported from the KML/KMZ file to a blue symbol. The red symbol is the one collected at Step 1. Now we have four red points and four blue points in the Adobe Illustrator document.
Step 3c) Adjusting the position of blue points using MAP View Editor Preview
As demonstrated above with three different examples, now you have to "guess" what would be the best option to do in order to match the four red and four blue points. Take a look at the points and see how they are distributed. Once you think you have an idea, open the MAP View Editor for the blue points.
Double-click the MAP View "my points in Google Earth"to open the MAP View editor dialog box.
Under the preview window in the MAP View edior dialog box, there are several preview options: colored boxes, thumbnails, and colored boxes and thumbnails. Click the "Configure Preview" button next to the drop-down list with the preview options.
In the Configure Preview dialog box, you can select which layers are displayed in the MAP View Editor preview and you can select the quality option for the preview. We will set the Thumbnail quality as Very High, then ensure that the points layers for the red points (layer name "my point") and for the blue points (my points in Google Earth) are selected.
Any time georeferenced objects need to be moved (points from the Google Earth in this case), use the MAP View editor to maintian georeferencing. If you do not use the MAP View editor to adjust the position of spatial objects, you will incorrectly change the georeferencing. When moving the objects (blue points in this MAP View), click the Move Tool in the MAP View editor, then drag the MAP View with the blue points. Once satisfied, click OK in the MAP View Editor to confirm your changes. The blue points in Adobe Illustrator will be moved as specified in the MAP View Editor window.
If you want to change the map scale, change the scale value in the MAP View editor. You will see the preview for the change in the scale value.
Step 3d) Transforming (reprojecting) a MAP View to another coordinate system
To transform (reproject) the MAP View of the blue points to another coordinate system, use the MAP View editor. In the MAP View editor dialog box, click the option Perform Coordinate System Transformation.
Click the Specify button and it will open the Specify Destination Coordinate System dialog box. Navigate to the appropriate destination coordinate system.
Once you have selected the destination coordinate system, its name will be indicated under the Perform Coordinate System Transformation section. Unlike Step 3c for scaling and repositioning the MAP View, the result of the transformation (reprojection) will not be reflected in the preview window except for the extent of the MAP View (you will see it if the preview option "colour box" is selected). Click OK to start the transformation (reprojection).
You will have to try Step 3c and 3d until you satisfy the position of the red and blue points. Now you have an important piece of information for making your map georeferenced. When georeferencing (see the previous blog for the steps), you can use one of the red/blue point as a MAP anchor point.
How do I know of the coordinate sytem or projection of the map if transformation (reprojection) is required?
This is a very common question and this is a great opportunity to explore the projections and coordinate systems used for making maps. For this example, the original map is produced for the nautical navigation. We started to think about the common projections used for the nautical navigation. The Mercator projection is a standard use for navigations and is the reason why we selected it as the destination coordinate system when transforming the MAP View.
If your map is a local scale, try to use the one commonly used in the region. For example, if the map is from a city in the USA, your map may be projected in one of the state plane coordinate system (take a look at this blog . You can find to see which state plane system is used in the region from this map). Also, many countries have their own grid system. You might want to search what kind of projection is used for the grid system.
If you have any question, please feel free to contact us (Avenza Support page).
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