Using a coordinate reference system

A coordinate reference system (CRS) is defined by the European Directive 2007/2/EC as "a system for uniquely referencing spatial information in space as a set of coordinates (X, Y, Z) and/or latitude, longitude, and height, based on a geodetic horizontal and vertical datum".

Note

For a short introduction of coordinate reference systems (CRS definition, conversion and transformation), please refer to the following URL:

http://www.crs-geo.eu/nn_124210/crseu/EN/CRS__Overview/crs-overview__node.html?__nnn=true

In this section, we will explore five coordinate reference systems (CRS) that are based on different horizontal datum. The terms coordinate reference systems and geographic coordinate systems are used interchangeably by the Esri's software.

Follow these steps to start exploring the datum and coordinates reference system with the ArcMap application:

1. Start the ArcMap application to open an existing map document. In the ArcMap -Getting Started dialog, select Browse for more and select WorkingWithDatums.mxd from <drive>:\LearningArcGIS\Chapter2\Datums.

2. The Table Of Contents section contains a single data frame called Datums with three layers. The WorldGrids_10 layer shows the graticule of parallels and meridians at 100 intervals. The ReferenceLines layer displays the reference lines: mean equator, and zero meridian. The Populated_places(Natural Earth 1:50m) layer contains the major cities and towns in the world at the scale: 1:50,000,000.

3. In Table Of Contents, right-click the Datums data frame, select Properties, and click the Coordinate System tab. You can see the parameters of the current coordinate reference system, which is a geographic coordinate system called WGS_1984, as shown in the following screenshot:

Tip

To read more about the D_WGS_1984 datum, please check the website, http://www.epsg-registry.org. Use the well-known ID of the coordinate reference system WKID: 4326.

4. At the bottom of coordinate system list, you can see a folder called Layers. This folder contains the list of the coordinate systems used by the layers in the data frame. All three layers share the same Geographic Coordinate System as the data frame.

5. You can bookmark your most-used coordinate reference systems by adding them in the Favorites folder. Use the Add To Favorites tool by right-clicking WGS 1984 or by selecting the yellow star. To delete a coordinate reference system from the Favorites folder, right-click it and select Remove From Favorites.

6. Click the General tab. As you may notice from the status bar, the display units are Decimal Degrees, which are determined by the Data Frame's coordinate reference system (Units Map: Decimal Degrees). The ArcMap can display the coordinate's values, independent of the original map units. Change the display units to Degrees Minutes Seconds by clicking on the drop-down list after navigating to Units | Display. Click on Apply and then OK to close the Data Frame Properties window.

7. From the Customize menu, select ArcMap Options and click the Data View tab. For Round coordinates to in Coordinate Display In Status Bar, choose 3 decimal places. Click on OK.

Now, let's add a little bit of color in the background from ArcGIS Online:

8. You need an Internet connection for this step. To check whether ArcGIS for Desktop is connected to ArcGIS Online, right-click ArcGIS Connection Utility from the Windows system tray and select Test Connection Now to run the connection, as shown in the following screenshot:

9. On the Standard toolbar, from the drop-down arrow next to Add Data, select Add Data from ArcGIS Online. Search for the Ocean data, select Ocean Basemap (Mature Support), and click on Add. Inspect the data using the Zoom In, Zoom Out, Pan, and Full Extent tools that are located on the Tools toolbar.

We will now customize the list of scales that are displayed in the drop-down list, as can be seen in the following screenshot:

10. You may already know that the scale is the fraction between the distance on a map and the true distance on the Earth. The scale is given as a fraction or a ratio: 1 millimeter [on the map] / 50,000 millimeters [on the ground] or 1:50,000. If we change the unit of measurement on both sides of the fraction, the scale remains same: 1 inch / 50,000 inches or 1: 50,000.

11. From the Standard toolbar, click the drop-down list of map scale and select <Customize This List...>. Type 50,000 and click the Add button to add the value to the map scale list. Again, add the 150,000,000 scale to the list. Click on OK to close the Scale Settings window.

12. As the Ocean Basemap layer does not provide enough details for a large-scale map display such as 1: 25,000, we will set a visible scale range for it. Select the 1:150,000,000 scale from the map scale list.

13. In Table Of Contents, right-click the Ocean Basemap layer and navigate to Visible Scale Range | Set Minimum Scale. Again, select the 1:50,000 value from the map scale list. Right-click the Ocean Basemap layer and navigate to Visible Scale Range | Set Maximum Scale. The layer will be drawn starting from the small-scale 150,000,000 to the large-scale 50,000.

14. For Scale, type 150,000,000 and press the Enter key. Inspect the results. If you set the map scale at 1:150,000,001 and 1:49,999, then the Ocean Basemap layer will not display and the check box from the left-hand side of the layer is dimmed in Table Of Contents.

15. If you want to see the layer at all scales, you have two ways:

• Right-click the Ocean Basemap layer and navigate to Visible Scale Range | Clear Scale Range.
• Right-click the layer and select Properties. In the General section, check the option, Show layer at all scales, and click on OK.

Tip

You can choose the display format of the scales: absolute and relative. The absolute scale is given as 1:6,000. The relative scale is given as 1 inch equals 500 feet. You can set the display format from the drop-down list of map scale by navigating to <Customize This List...> | Scale format | Edit. ArcGIS automatically converts the relative scale to an absolute scale.

We will now learn how to add the latitude and longitude (geographic coordinates) expressed as sexagesimal degrees, minutes, and seconds (DMS) in the attribute table of the Populated_places (Natural Earth 1:50m) layer, as shown in the following screenshot:

Follow these steps to add the latitude and longitude values for the Populated_places (Natural Earth 1:50m) layer:

1. In Table Of Contents, right-click the Populated_places (Natural Earth 1:50m) layer and click Properties. Select the Source tab. You will find out that the ne_50m_populated_places.shp shapefile is the data source of the layer. The coordinate system of the data source is the same geographic coordinate system called GCS_WGS_1984. Click on OK to close the Layer Properties window.

2. Right-click the Populated_places layer again and click Open Attribute Table. Examine the attribute fields of the layer.

3. All feature geographic coordinates are stored by ArcGIS in decimal degrees. In this step, we would like to store the geographic coordinate as Degrees Minutes Seconds (DMS) in an attribute table using two new fields. Click the Table Options button and select Add Field. For Name, type LAT_DMS; for Type, select Text from the drop-down list. Click on OK. Use the horizontal scroll bar to see the field (last column). Right-click the heading of the LAT_DMS field and click Calculate Geometry. Click on OK to calculate the attributes outside of the edit session and check the Don't warn me again option. In the Calculate Geometry dialog, for Property, select Y Coordinate of Point. For Units, select Degrees Minutes Seconds (DDD MM'SS.sss"[N|S]). Uncheck the Calculate selected records only option. Click on OK.

4. Repeat the last step to create the LONG_DMS field that stores the longitude using Property: X Coordinate of Point and Degrees Minutes Seconds (DDD MM'SS.sss"[W|E]). If you want to change the order of the fields in this table, click the heading of the column and drag it to where you want it to be placed.

5. Move and resize the Table window in the ArcMap window so that you can see the map display area. From the Tools toolbar, use Select Features to select a city from the map display area. At the bottom of the Table window, click Show selected records to view only the selected city.

6. To see all records from the attribute table again, click the Show all records button. To unselect the city use the Clear Selection button at the top of the Table window from the Table Options menu or use the Clear Selected Features tool, which is next to the Select Features tool. Leave the Table window open.

In the next steps, you will add three shapefiles with different geographic coordinate systems than the data frame and one shapefile with an unknown coordinate system without choosing a suitable datum transformation. By ignoring the datum transformation, we want to emphasize the horizontal datum shifts and how the geographic coordinates of a given point are different depending upon the geodetic datum, as shown in the following general figure:

7. From the Standard toolbar, click the Catalog button. In the Catalog window, go to <drive>:\LearningArcGIS\Chapter2\Datum. Right-click ne_50m_populated_places_GRS80.shp and select Properties. Click the XY Coordinate System tab to check the associated coordinate system. The geographic coordinate system is GCS_ETRS_1989 with the D_ETRS_1989 datum that is based on the spheroid called GRS_1980. Click on OK to close the Shapefile Properties window.

8. Click and drag ne_50m_populated_places_GRS80.shp into the map display. Geographic Coordinate Systems Warning displays, as shown in the following screenshot:

9. ArcGIS needs to change the layer's datum through a datum transformation. If you close Geographic Coordinate Systems Warning, no transformation will be applied and the data will be a simple draw. Read the message carefully and leave the last two options unchecked. Even if we don't want to consider how the datum will be transformed in this step, we still want to be warned every time we add a new layer with a different datum into the map display. Click on Close to dismiss the warning.

Tip

If you accidentally check one of these options and want to display it again, go to <ArcGIS_HOME>\Desktop10.4\Utilities and double-click AdvancedArcMapSettings.exe. Select the TOC/Data tab and uncheck the following options: the Skip datum check and Skip 'Unknown Spatial Reference' warnings. Some of the options in the ArcMap Advanced Settings utility require administrator permissions. Make sure that your ArcMap application runs "As Administrator".

10. Repeat the last three steps to add the ne_50m_populated_places_European1950.shp and ne_50m_populated_places_North_American1927.shp shapefiles.

11. Add the last shapefile called ne_50m_populated_places_Pulkovo42.shp into the map display. A new warning message displays and tells you that spatial reference information is missing. Just because the dataset is missing a spatial reference does not mean it is not in a coordinate system. It just means that it has not been assigned or defined for the dataset so that ArcGIS can understand it and apply it to a proper on-the-fly transformation. Click on OK.

At your current display scale (around 125,000,000), your shapefiles seem to perfectly overlap. We will take a closer look at your map and examine the difference between the point features represented by the fifth spheroids.

12. From the Bookmark menu, select Detail 1- Paris (France) to see a city at scale 1: 0.1, as shown in the following screenshot:

13. From the Tools toolbar, click the Measure tool and set its Distance to Meters. Measure the distance from the point represented on the Clarke1866 spheroid to the point represented on the WGS1984 spheroid. The difference between these two points should be around 197 meters. Depending on where you clicked, your value may be slightly different from the one shown in the previous screenshot.

14. Use the Select Features tool that is located on the Tools toolbar to select the five visible city points. Once selected, open Attribute Table for each layer by right-clicking the layer name in Table Of Contents and selecting Open Attribute Table. Click the tabs located at the bottom of the Table window to view the data associated with each layer. Depending on the geodetic datum, the geographic coordinates for the same city are different at the seconds' level.

Tip

To arrange the tables within the Table window so that each table is visible at the same time, click the tab of each table and drag it into the right-docked position. You can use the Arrange Tables commands on the Table Options icon, which is located at the upper-left corner of the Table window to change the place of an active table.

In the upcoming steps, we will choose the optimal on-the-fly datum transformation in order to fix or minimize the overlap problem. Follow these steps to start working with the ArcMap's on-the-fly datum transformations:

1. First, we will define the coordinate system for ne_50m_populated_places_Pulkovo42.shp. In the Catalog window, right-click ne_50m_populated_places_Pulkovo42.shp and navigate to Properties | XY Coordinate System. To select the current coordinate system, navigate to Geographic Coordinate Systems | Europe and select Pulkovo1942. Click on Apply and OK. In Table Of Contents, right-click the ne_50m_populated_places_Pulkovo42 layer and navigate to Properties | Source to check whether the geographic coordinate system was updated. Click on OK.

To correct the ArcMap's on-the-fly datum transformations, we have two options:

1. Remove the layer and add it again.

2. Use the Data Frame Properties dialog.

In the next step, we will use both of these methods.

2. Remove the ne_50m_populated_places_Pulkovo42.shp layer from Table Of Content (right-click the layer and click Remove).

3. From the Catalog window, drag it again into the map display. The Geographic Coordinate Systems Warning window will pop up. Click the Transformations button, and for the Using (choices are sorted by suitability for layer's extent) options, click the drop-down arrow and select the first transformation in the list: Pulkovo_1942_To_WGS_1984_16. Click on OK and Close. The ArcMap's datum transformation has reduced the point location shift. Unselect the features with the Clear Selected Features tool and zoom in close to the WGS84 point to take a closer look. Most probably you will need to zoom to a scale of 1:1.

Tip

Before the use of the first option from the transformation drop-down list, we recommend that you confirm the suggested transformation by quickly studying the Esri document called geographic_transformations.pdf stored at <ArcGIS_HOME>\Desktop10.4\Documentation. This file lists all the supported datum transformations and areas for which they are suited.

Based on the evaluation of data extent and given datums, ArcGIS for Desktop will first list what it considers the most accurate transformation to use. However, the one ArcGIS recommends first may not be the best for your particular data. You may need to try multiple transformations before finding the one that works best with your data.

The information related to the selected transformation will be stored by ArcMap as part of Data Frame Property. This information will be used by your data frame when you will add another dataset having the same datum called D_Pulkovo_1942. Try to add ne_50m_populated_places_Pulkovo1942.shp again, and you will notice that a new layer is added to Table Of Contents and displayed without any other geographic transformation warning.

In the succeeding steps, we will try to improve the datum transformation for the rest of the shapefiles.

4. From the Tools toolbar, click the Go Back To Previous Extent tool to return to a best previous map scale. In Table Of Contents, right-click the Datums data frame and click Properties. Select the Coordinate System tab. Click the Transformations button to open the Geographic Coordinate Systems Transformations dialog, as shown in the following screenshot:

5. For Convert from option, select GCS_ETRS_1989. Leave Into: GCS_WGS_1984 unchanged. For the Using options, click the drop-down arrow and select the first transformation in the list: ETRS_1989_To_WGS_1984. Do not click on OK.

6. Repeat the last step to choose a datum transformation for GCS_Europe_1950 and GCS_North_American_1927. Click on OK and Close. Move the Data Frame Properties window in the ArcMap window so that you can see the map display area better. Click Apply and notice the results. Click OK to close the Data Frame Properties window.

7. Again, zoom in close to the point named WGS84 to take a closer look. Use the Measure tool to measure the small differences between these points.

Please remember, ArcMap will temporarily calculate the coordinate values corresponding to the data frame's coordinate system through an on-the-fly datum transformation only for display purposes. This means that the coordinate reference system and stored coordinate values of the displayed shapefiles remain unchanged. If you want to permanently change the coordinate reference system of the shapefile, you would need to use the Project tool from ArcToolbox | Data Management | Projections and Transformations to project the data.

8. Save the map document at <drive>:\LearningArcGIS\Chapter2\Datums as MyWorkingWithDatums.mxd. Click on File and select Exit to close ArcMap.

You can find the results of this exercise at <drive>:\LearningArcGIS\Chapter2\Datums\Results.