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Projections in ArcGIS

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I have collected tracks using a GPS for a city in Morocco and I have the same data that i have digitized from a different source with the local projection. I do all the necessary steps required by ESRI and I still do not get the data to match. It gets to be close but still far by many Kilometers. Could anyone give me some tips or ideas on the best way to reproject (in ArcMap8.1) data from

WGS84 to Lambert Morocco North (which is equivalent to Ellipsoïde de référence: Clarke 1880 :
a=6378249,145 m
b= 6356514,869m

Fundemental point: Merchich with the following criteria
ϕ =37G 1665’’ 654
λ = 8G 3973’’ 133
h = 243M42
If anybody can help me out I can send samples of the data from both sources.


Martin Gamache

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I do all the necessary steps required by ESRI

What is your workflow?

1. What GPS, what proj/datum and format is native data. I would assume Lat/long WGS84?
2. What process to go from GPS to ARC format? What format shapefile, geodatabase, etc...?

3. What steps to convert from native projection to desired projection?

The way you move your data from the unit to ARC and how the projection is handled in the process can make a big difference. If you are using a Garmin or Thales unit I can probably take a look at it and also give you some insight into what may be happening if you provide a bit more info. Otherwise....

You may also want to display both your digitized data and your GPS data with reference data of a known reliable position to determine which is off. It may be your digitized data which is the problem...

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