GIS data export

[Anonymous]

In using GIS data and AutoCAD Maps 3d I have the following issue.

The data is recorded in a GPS device with the coordinate format ARC 1960.

After importing the data in AutoCAD maps 3d and assigning it to ARC 1960 (EPSG code: 21036) I export the data as a KML file.

Looking at the KML file from the device and from AutoCAD maps 3d I get a difference of approx 12 m. This can be seen in Google Earth.

Why is this so? What do I need to do, that AutoCAD maps 3d does not shift the data?

Best,
Klaus

[АлексЮстасу]

Hi, Klaus,

The error in the position of satellite images in Google Earth is about 8-10 m. This can be seen by turning on the History mode, looking at images from different times.
You need to compare Map 3D data with data of exact tools - QGIS, Global Mapper or the like.

[ChicagoLooper]

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@Anonymous wrote:

In using GIS data and AutoCAD Maps 3d I have the following issue.

The data is recorded in a GPS device with the coordinate format ARC 1960.

What is the PRECISION of the coordinates that you are supplying to Map 3D?

It sounds like your GPS coordinates are in lat/long and you do not have enough decimal places. Increase the decimals in both latitude and longitude to a minimum of 6 decimal places then export to KML again.

[Anonymous]

Hello,

thank you so much for the reply! Unfortunately both ideas are not bringing the result.

1. Accuracy of google earth. Yes, this is considered. But it is not the issue. The case is if I export from the GPS logger directly to KML there should be not a difference as compared to importing it at AutoCAD Maps3d and exporting it to KML

2. I tested the decimals. But the result is not improving.

When importing the data in Autocad it is not assigned to a coordinate system. I guess when I assign the coordinates to ARC1960.UTM-36S (EPSG code: 21036) the coordinates are interpreted in a different way. 

However, the google format is WGS84. Using WGS84 at Autocad shifts the data to an other part of the world.

Does somebody have an idea how to assign it correct?

[ChicagoLooper]

If you want to export to Google Earth Pro then a more appropriate match UTM-84-36S or EPSG 32736. Start over, then replace EPSG 21036 with EPSG 32736. Once you have performed the replacement, run the export again.

[Anonymous]

Now I am very close to the right result.

I had a missmatch between WGS84 and ARC1960.

The right way of doing it is to export at my GPS logger with the format WGS84 to the format dxf. In Autocad I need to assign the format UTM84-S36 (EPSG code: 32736).

The result is now a difference of 1 - 2 meter. This looks like it is an rounding failure.

I can change the accuracy at AutoCAD via DrawingUtitlty -> Unit.

However, this does not change the accuracy of the output KML file. Is it possible to change here the decimals?

[ChicagoLooper]

I would change the 'precision' in the GPS unit, not the dxf. Your dxf is only as accurate and precise as the source, which in your case is the data collected using GPS. If your GPS is not accurate then neither is your output.

To fix your issue, you need to look at the data that was collected by the GPS.

• What does the data from the GPS look like?
• Are you dealing with point coordinates? Lines? Polygons?
• Are you dealing with latitude/longitude? Or UTM meters?
• Can your GPS logger output formats besides dxf? What are those formats?

If you can answer these questions, then you might be able to find a resolution to your issue. If you ignore any of the questions, you'll likely be dealing with accuracy +/-2meter.