KML Data Integration

From ProStar Geocorp

This topic provides information on KML data integration from PointMan to external GIS systems.

Geodetic Reference Systems in Google's KML File Format

KML (and Google Maps) uses plain, un-projected WGS84 (EPSG 4326) decimal Lat/Long coordinates. For its reference system, KML uses 3D geographic coordinates: longitude, latitude and altitude, in that order, with negative values for west, south and below mean sea level if the altitude data is available. The longitude, latitude components (decimal degrees) are as defined by the World Geodetic System of 1984 (WGS84). The vertical component (altitude) is measured in meters from the WGS84 EGM96 Geoid vertical datum. If altitude is omitted from a coordinate string, e.g. (-77.03647, 38.89763) then the default value of 0 (approximately sea level) is assumed for the altitude component, i.e. (-77.03647, 38.89763, 0).

PointMan Output KML File Integration with External GIS Systems

Open source and commercial GIS software converting KML to shapefiles will assume KML data's source coordinates to be in WGS 84 (EPSG 4326). As such, the KML conversion to shapefile will be generated in the WGS84 coordinate system. This implies that regardless to what coordinates/datum is used to collect and store data in KML format, converting it to shapefiles will result the output to be in WGS84 coordinate system.

Integration of KML File from PointMan with External GIS Systems

Data Collected in PointMan does not undergo any coordinate transformation. The coordinate system/datum utilized for data collection is stored as-is and exported as-is to KML file. For example, data collected in ITRF2008 datum is stored as-is in PointMan and exported as-is to KML. However, due to KML's limitations to support coordinate/datum definitions other than WGS84, while converting KML to shapefile GIS software will treat this data to be in WGS84. To get the data back to ITRF2008 (or any user defined coordinate reference), users need to redefine coordinate system definition back to ITRF2008. The steps for redefining the coordinates is using ESRI ArcMap is outlined below.

Defining a Projection/Coordinate System in ArcGIS using ArcToolbox

To define a projection for a shape file, users shall use Arc Toolbox. Users also need to have write permission to the file to do this. The process described below will add/modify a new or existing file with the extension .prj to the GIS data layer that must accompany the rest of the files that go into a shape file. The Define Projection tool works both from the ArcCatalog and the ArcMap interface. Users can have the file open in ArcMap while using the tool in ArcMap. However, users CANNOT have the file open in ArcMap while using the tool in ArcCatalog. Use one interface or the other, but cannot have the file open in both!  

  1. Open Arc Toolbox (in ArcMap or ArcCatalog, choose Window – ArcToolbox or click on the red Toolbox icon).   
  2. Go to Data Management Tools – Projections and Transformations and click Define Projection. 
  3. In the Define Projection dialog box under Input Dataset or Feature Class, click the folder icon to navigate to the folder where your shape file is located. Once located, click Add. If you are in ArcMap and the shape file is already displayed, you can simply click on the little arrow and choose the open file.   
  4. In the Coordinate System box, click on the icon to the right. The Spatial Properties dialog box displays.   
  5. There are two common methods for selecting a coordinate system:
    • If you have other files on the computer with the target coordinate system already defined, you can click on the Import button, navigate to the defined shape file and have it use that same coordinate system for the undefined shape file as well (find it and click Add).
    • Or press Select..., and choose the coordinate system to apply.  For example, we would choose Geographic Coordinate System - GCS_ITRF_2008, WKID: 104257 Authority: ESRI- ITRF 2008..., then click on Add.   
  6. Users will see a yellow warning icon that the layer already has a projection defined. 
  7. This is expected
  8. Click OK.  Remember we are redefining the projection from KML's incorrect WGS84 to the right user defined IRTF2008.