Community

Map projection, data frame and coordinates system

1785
1
10-25-2016 07:21 AM
francescarusso
by
New Contributor III
‎10-25-2016 07:21 AM

Hi all,


I am writing to ask some help/clarifications about data frame, coordinates and projections. I do not use GIS very often and I have a big gap in cartography. I use ArcMap 10.2 Desktop someone sent me a layers package. I imported the layers into a new empty document. The data frame now gets the layer's proj. coordinate system and projection. -1) some layer in the layers pack have BRITISH NATIONAL Grid coord system and Projection:Transv.Mercator and other layers have GCS WGS84 Datum D_WGS84. Shall I have to change anything? Are they correct or because they have different coord system I will have problem (i.e doing some measurement or analysis?) - 2) If I add some raster Admirality multichart a message for transformation comes up. It asks me if I want to transform the coordinate system as some of the charts are in ETRS_1989 Mercator and some are ETRS 1989 Transv Mercator. Shall I have to transform? If so, does it results in any measurement distortion or conflict with the layer package? -3) I had to create a new feature class so I used the BRITISH NATIONAL GRID Transv.Mercato because of the Data Frame.Am I correct? Does it result in any measurement or spatial analysis problem? -4) I tried to edit my new feature class and a kind of info box comes up with "Spatial reference does not match data frame" related to the layers which were in GCS WGS 84 instead of British National Grid. Is this a problem? Thank you very much in advance to who will have time to clarify. Have a nice day Francesca

0 Kudos
1 Reply
MelitaKennedy
by Esri Notable Contributor
Esri Notable Contributor
‎11-21-2016 10:03 AM

Hi,

I'm sorry that I didn't reply earlier.

Note: Pro is different; will explain below after ArcMap info.

ArcMap will reproject all layers that have coordinate systems that aren't the same as the data frame's. What it won't do is the geographic/datum transformation part if there are no transformations already set. Let me show the workflow.

GCS = geographic coordinate system (datum); PCS = projected coordinate system

Data frame in PCS2 based on GCS2

Layer in PCS1 based on GCS1

  1. PCS1 is unprojected to GCS1
  2. GCS1 is transformed to GCS2 (if transformation is set, otherwise it's internally redefined to GCS2)
  3. GCS2 is projected to PCS2

If a layer is in GCS1 already, then just steps 2 and 3 occur.

To check on whether there are transformations set,

  1. Open the data frame properties (right click Layers, then select Properties)
  2. Click the Coordinate System tab
  3. Click the Transformations button
    1. In the top box is the unique GCS in the layers and data frame
    2. Below that is the data frame's GCS
    3. At bottom is a pull-down that may show the name of a transformation or "None".
  4. Select a GCS in the top box, then check the pull-down to see if a transformation is set or not.
  5. We show a sorted list based on extent and accuracy, but it can be fooled by having a world base map, etc

How much data can be off depends on the two GCS involved. Two newer GCS like WGS84 and ETRS89 should agree within a meter or so (or better). British National Grid uses OSGB 1936 so it will differ by more like a hundred meters from ETRS89 or WGS84.

Pro: Uses a lookup table to automatically set the geographic transformations. The lookup table usually uses the most generic transformations available which sometimes are not the best transformation for a particular area. Just as in ArcMap, you can open the properties of a map and change the transformation.

Melita

0 Kudos