Best Practices For Displaying Coordinate Data To Avoid Misinterpretation
Understanding Coordinate Reference Systems
A coordinate reference system (CRS) defines how latitude, longitude, and height positions are displayed on a map or model of the Earth. Defining the CRS requires setting the map projection, which represents the 3D earth surface on a 2D plane, and the datum, which models the size and shape of the earth.
Defining Map Projections and Datums
Map projections geometrically transform the earth’s 3D shape into a 2D coordinate plane. Some preserve shape, some preserve area, some preserve distance and bearing. No projection maintains all properties. The datum defines the model of earth’s size, shape, and orientation in space. Hundreds exist to best fit regions. Choosing the appropriate projection and datum for displaying coordinate data reduces distortion and misrepresentation.
Common Coordinate Reference Systems
Widely used geographic coordinate reference systems (lat/long) include WGS84 and NAD83. Common projected systems (x/y) include UTM, State Plane, and Web Mercator. Data collected in different CRS may need transformation for accurate combined analysis and display.
Displaying Latitude and Longitude Values
When displaying geographic coordinates, the common formats are degrees-minutes-seconds (DMS) and decimal degrees. Precision and units must be considered to avoid misinterpretation.
Degrees, Minutes, Seconds Format
The DMS format represents latitude and longitude with component parts for degrees, minutes, and seconds. For example, 45°30’15″N. It allows variable precision but can cause confusion when degrees, minutes, or seconds are truncated or units omitted.
Decimal Degrees Format
The decimal degrees format represents latitude and longitude with a single numeric value, maintaining decimal places for precision. For example, 45.50417. This avoids truncation issues but the appropriate display precision must still be determined.
Setting Display Precision
The level of decimal precision depends on data accuracy and intended use. Low precision coordinates like 40°N, 105°W identify regional areas. Precision to thousandths or ten-thousandths of a degree provides accuracy for local areas and features. Consistently display precision and decimal places to avoid misalignments.
Handling Coordinate Transformations
Transforming coordinates between datum-based geographic coordinate systems or to/from plane-based projected systems requires specialized algorithms to maintain data integrity and accuracy.
Transforming Between Geographic and Projected Systems
Converting data from latitude/longitude to map coordinates requires projecting it based on defined parameters. The inverse projection transforms it back. This can introduce distortions so certain projections best represent specific areas. Consistently use appropriate projections.
Managing Datum Shifts
Since datums reference different earth models, transforming between them requires calculating offsets. Small shifts along all axes maintain integrity across the dataset. Identify source and target datums, choose proper transformation methods to prevent feature misalignment.
Best Practices
Following standards and guidelines when displaying coordinates reduces interpretation issues for analysis and decision-making.
Labeling Axes and Units
Always label map axes and coordinate readouts with system, units, and precision to provide essential reference information for proper data interpretation.
Choosing Appropriate Precision
Display coordinate precision suitable to data accuracy and intended use scale. Lower precision for regional analysis, higher precision for local mapping. Standardize across organisations and applications.
Using Decimal Degrees for Web Maps
Web maps and mobile applications typically display location data in decimal degrees, avoiding DMS complexity. This provides sufficient precision for general use with efficient processing.
Transforming Coordinates to Match Data Frame
Projected coordinates inherit their CRS from the overall map frame. Transform new layered data to match or misalignments occur. Identify CRS for each dataset and perform required datum and projection shifts.
Example Code
Development languages provide functions for managing coordinate data representations, precision, transformations, and projections.
Python – Setting Display Format for Decimal Degrees
import geopy.distance
lat = 45.5045
lon = -122.6743
print(f"{lat:.4f}, {lon:.4f}")
# Prints coordinate values to 4 decimal precision
JavaScript – Converting DMS to Decimal Degrees
function convertDMSToDD(degrees, minutes, seconds, direction) {
let dd = degrees + minutes / 60 + seconds / (60 * 60);
if (direction == "S" || direction == "W") {
dd = dd * -1;
}
return dd;
}
let lat = convertDMSToDD(45, 30, 15, "N");
let lon = convertDMSToDD(122, 40, 35, "W");
SQL – Transforming Geometry Between Coordinate Systems
UPDATE data SET geom = ST_Transform(geom,4326) WHERE ST_SRID(geom) = 26915;
The above transforms all geometries in data from projection SRID 26915 to geographic WGS84.
Troubleshooting Common Issues
Despite best practices, misalignment, overlap, and shift errors still occur. This section explains common causes and solutions.
Overlapping Features After Reprojecting
Projecting data into planar coordinates can cause feature overlaps due to spatial distortions introduced. Use appropriate projection minimizing distortion for target display or analysis area.
Shift in Feature Locations from Datum Differences
Datums reference different spheroid models of earth causing shifts in transformed coordinates. Calculate and apply datum transformations based on the specific datums and regions involved when projecting between geographic systems.
Truncating Coordinate Values Causes Misalignment
Consistently maintain precision and decimal places throughout a workflow to avoid truncating coordinate values. Display complete values with standardized formatting to align map data.
