Hi,

I am wondering if someone can shed some light on a small problem.

I have got three sets of data that I need to use in a hydrological model and they need to be in the same projection. There is a DEM and soil grid file in Geographic co-ordinate GCS_WGS_1984 and land use data in WGS_1984_Albers. I thought it would be a good idea to use the transformation tool to make DEM and soil both WGS_1984_Albers. But now the maps do not line up- if I can call it that; for instance they do not overlap now in the model.

Thanks for any help,

Kim

I am wondering if someone can shed some light on a small problem.

I have got three sets of data that I need to use in a hydrological model and they need to be in the same projection. There is a DEM and soil grid file in Geographic co-ordinate GCS_WGS_1984 and land use data in WGS_1984_Albers. I thought it would be a good idea to use the transformation tool to make DEM and soil both WGS_1984_Albers. But now the maps do not line up- if I can call it that; for instance they do not overlap now in the model.

Thanks for any help,

Kim

Coordinate transformations usually referes to transforming the underlying datum,

not reprojecting the coordinates.

In this case, your datums are the same (WGS 1984),

so no transformation should be required, just a projection.

Conceptually, features are projected, not from the earth, but from a generalized figure (a globe)

The datum defines the globe.

Using differently defined globes (datums) can lead to very different locations for features,

so transformation from one dataum to another can be significant.

It is true that a projection 'transforms' the globe onto a plane

(sometimes with intermediate steps such as a developable surface (a cylinder or cone)

or with rotaions or skews...), but we usually refer to it as projecting rather than transforming.

How far apart are the locations that should be coincident?

In what direction do they lay?

and most important:

What exactly did you do to each of the (geographic coordinate) data sets?

(tools and parameters, please)