Projecting a Map

This topic assumes all we want to do is project a map using drawings or images that have been imported into Manifold correctly. When using drawings or maps in Manifold format or other modern GIS format that correctly saves information about projections there is no need to understand the internal workings of projections. There are also few difficulties when we import unprojected data.

If you are struggling with the import of a projected map (say, in UTM or State Plane Coordinate System) using data in some ancient format such as ESRI .shp, please jump to the Projections and Legacy Formats topic and then follow the links to all the ugly stuff you must consider.

To project a map, we take note of some map characteristics and then we use the Edit - Assign Projection dialog.

Step 1: Note the Center of the Map

Most projections require a center for the projection. That's the latitude / longitude location at the center of the region to be projected. Before launching the Edit - Assign Projection dialog we should note the center and some other parameters in case we want to set projection parameters manually. Manifold has a Suggest button in the projection dialog that will set parameters for us but it is often helpful to note them in advance so we can compare the suggested parameters to what we might choose to set on our own.

To determine the center of the map move the mouse cursor to the center of the region and note the latitude / longitude numbers given in the status bar. For larger regions, round up to some reasonably even number. For example, a reasonable center for maps of the US made in Lambert Conformal Conic is 40 degrees latitude and -100 degrees longitude.

Step 2: Choose the Right Projection

Launch the Edit - Assign Projection dialog and choose the projection desired. The projection to use depends mainly on the region to be mapped and the intended purpose of the map. The suggestions below work well in almost all cases and are based on the region to be mapped.

There are so many projections available within Manifold that they are organized within a tree diagram. All of the following are in the Standard category. The subcategories are noted in parentheses where the name of the projection itself does not make it clear to which subcategory it belongs.

Most users will use only a few projections over and over. For general use, the Manifold team recommends the following:

Other than personal taste in visual appearance there are only three reasons not to use one of the above projections:

·      A specific projection is required to match other data available or to exchange data with other users.

·      The technical characteristics of a particular projection such as preservation of scale or relative area are required for creating a paper map or other presentation.

·      A specific technical characteristic, such as datum used, is not a good match to the data.

As an example of the latter point, one limitation of Orthographic is that the projection is defined only for the sphere datum. If other datums are to be used, an alternative "view from space" projection is the Stereographic projection.

Manifold provides a very wide array of projections in addition to the standard projections mentioned above.

Step 3: Project the Map

In the Edit - Assign Projection dialog specify the projection selected using the tree diagram and the center for the projection and other parameters. When specifying latitudes and longitudes remember that west longitudes are negative numbers and that south latitudes are also negative numbers.

Some projections will require other parameters. For example, when using Conic projections two "standard" latitudes (also known as "standard parallels") are required. These should be about one fifth of the way down and one fifth of the way up the map for greatest visual fidelity, but most people will round these values to memorable numbers. For the US, many people use 35 degrees as the first standard latitude and 45 degrees as the second standard latitude. The Suggest button will automatically load reasonable values for standard parallels into the projection's parameters.

The Suggest button will cause the system to estimate the center of the map based on the size of the largest drawing rectangle that participates in the map. Manifold will then load the estimated, rounded center into the Center Latitude and Center Longitude parameters together with estimates of "reasonable" values for the other parameters. These may then be edited as desired.

There's no need to worry about the Datum unless we are exchanging projected data from old formats or are using Manifold to convert GIS data between different projections or datums. If we use World Geodetic 1984 (WGS84) as our standard datum we'll be in great shape to utilize GPS data and other modern instrumentation, almost all of which use WGS84.

If we need to match a particular projection used by other data or required for data exchange we must find out what datum is necessary and pick it from the list of datums available. Manifold includes virtually every datum that has been used anywhere in the world for mapping.

The projection dialog includes several options for use by experts. These are changed only in special cases, such as manual georegistration. See the Edit - Change Projection dialog topic.

Additional Notes on Choosing a Projection

For showing small regions of the Earth, such as a hiking trail within a park almost any projection is acceptable. Any distortion of shape, area or scale for such small regions caused by the projection will likely be less than the intrinsic accuracy of the data. In small regions almost all applicable, correctly centered projections are visually indistinguishable even to experts.

We suggest the Orthographic projection because it provides a natural appearance of mapped shapes from an overhead view. It is also the "default" projection used with images and abstract CAD drawings when they are first imported into Manifold. This will enable fast usage of images within maps when all use the same projection.

For mapping larger regions, such as countries or continents, learning about a handful of frequently used projections is enough. The main choice is between projections that provide better appearance and accuracy of East-West regions (like the US), North-South regions (like South America) or circular regions (like Antarctica or the Pacific Ocean). In almost all cases a choice between Lambert Conformal Conic (for East-West regions) or Orthographic (for South America or Africa) is a good choice. The Lambert Conformal Conic projection uses two additional standard latitudes as described in the Lambert Conformal Conic projection's topic.

For showing the entire world at once, there is no single projection that adequately minimizes distortion while preserving accurate representations of area or scale. Maps of the entire Earth are used almost exclusively for thematic presentation purposes and not for measuring purposes such as navigation. In such maps the main objective is a visual appearance that preserves in a subjective way the relative sizes of countries, with rough preservation of shapes.

Most users will gravitate toward one of the standard cylindrical or pseudocylindrical whole-world projections and will tend to use it over and over. It is almost exclusively a matter of personal taste. We suggest either the Robinson because it is currently a fashionable projection or the Miller Cylindrical because it provides straight lines for parallels and meridians. Many users will prefer the Mollweide or another choice.

The only projection for whole Earth use one should absolutely, positively avoid is the Mercator projection. The Mercator projection was intended for marine navigation. Its main positive feature is that it has straight rhumb lines, an essential requirement for marine navigation. As a whole Earth projection it is one of the worst possible choices due to the great distortion of regions not near the Equator.

Why So Many Projections?

If only a handful of projections are recommended, why does Manifold provide so many other projections? Manifold provides so many projections because many projections are in common use. To import data from those many different projections Manifold must understand them all.

Many projections are in common use because most mapping of the Earth was done before the era of fast and cheap computers. In earlier times paper maps were the living instruments used to measure the Earth. The specific technical characteristics of projected maps such as scale preservation are extremely important if paper maps using such projections will be used as measuring instruments. Different map projections were required for different countries and regions so that mechanical measuring and plotting on paper maps for different purposes could be easily achieved.

With modern computer software none of that is necessary any more. Measurements of distance or area within a GIS on a computer can be done with far greater precision than by using a straight-edge and compass on a paper map. As a result paper maps are less frequently used as precision measuring instruments. In modern times paper maps are used for presentation and for visual estimates of relationships.

There is no longer any reason to restrict our projection methods because they are easy to compute by hand. For presentation purposes computers can instantaneously create a custom-centered projection of a data set to suite whatever specific view is required. With computers and software like Manifold we can use relatively few different map projections and simply set the parameters so the projection is always a good match for the view desired. However, since the world's database of digital maps was created mainly from the legacy archive of paper maps the numerous different projections created for use with paper maps continue to be prevalent in the digital era.

This is not necessarily a negative thing, since different projections provide different artistic impression and even a different conceptual feel. Having a rich set of projections from which to choose is like having a nice collection of fonts to use in one's text documents. Different projections can be used to good effect for improving the visual appearance of presentations.

For more on choosing projections, see the Projections Tutorial , General Projections Concepts and the Guide to Selecting Map Projections .

For more on datums see The Earth as an Ellipsoid .

Re-projecting Drawings, Images and other Components

The above procedure describes re-projecting a map. To permanently re-project a specific component we open it in a window and apply the Edit - Change Projection command. If the component appears in a map and we would like to re-project it to use the same projection as the map, we can right click its tab in the map and choose Project to Map.

Differences between Assign Projection and Change Projection

·      Assign Projection appears both with map windows and with individual component windows. When used with map windows it changes the projection used in the map view without actually changing the data in the components. Map windows have dynamic projections that can show their contents in whatever projection is desired regardless of the native projections of what is inside them. When used with component windows, this dialog changes the projection assigned to be used to interpret the coordinate data within the component.

·      Change Projection appears only with individual component windows such as drawing windows or image windows. This command changes the actual coordinate data in the component into a new coordinate system. It can only work correctly if the starting projection assigned to that component was correctly assigned, so before we can use Change Projection to change a projection we must use Assign Projection to verify that the projection originally assigned was correctly assigned.

Problems with Drawings from Legacy Formats

Failing to import a projected drawing correctly will lead to unpredictable effects when the drawing is used in a map. If we import a projected drawing from a geographically unaware format such as .shp or .dxf we must tell Manifold the correct projection to use to interpret that data since the format does not save this important information. It is, of course, a blunder to publish projected drawings using formats such as .shp, but this is done all the time.

To import a projected drawing or image from a geographically unaware format:

1. Import the drawing or image using File - Import. Use default settings.

2. Open the drawing or image.

3. Use the Edit - Assign Projection dialog to specify the projection information that should be used.

The Edit - Assign Projection dialog allows us to manually provide the correct projection information into the coordinates properties. Manifold will then be able to make sense of the data just imported.

Do not confuse the use of a chosen projection view in a map window (via Edit - Assign Projection ) with the use of Edit - Assign Projection in a drawing window to specify projection information missing from legacy formats. If a projected drawing is imported from, say, .shp format and you fail to tell Manifold the correct projection to use, that drawing has not yet been correctly imported. Subsequent use of the drawing in maps may cause bizarre effects or lengthy delays as the system attempts to compute a new projection for the map based on fundamentally inaccurate coordinates.