Archive For The “Projections/Geodesy” Category

Ubuntu GIS from Scratch: Step 5 – Spatial SQL

Welcome to Step Five in our exploration of open source GIS using Ubuntu. In the previous installment, we uploaded some spatial data into a PostGIS. database. The next logical step is to install QGIS and add start visually analyzing this data, which we’ll get to in Step Six. But before that, I wanted to take [...]

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Ubuntu GIS from Scratch: Step 4 – Loading Data.

(Note – this post was updated on October 28 to include new information!) Welcome to Step Four in our exploration of open source GIS using Ubuntu. In the previous installment, we spatially extended our PostgreSQL database server with PostGIS. Now we’re ready to load some geographical data into our database. PostGIS includes a tool called [...]

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Student un-Projection Projects

Part of the GIS Day 2011 festivities at Texas A&M University included a student poster display in Evans Libary on the main campus.  There were 29 posters covering a wide range of GIS topics from various classes and departments.  All 29 posters contained a scalebar, a legend, and a North arrow.  But only three contained [...]

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A Difference of Projections

If you divide your time between ArcGIS and QGIS, you may have noticed that they don’t use the exact same projections.  For Northern Colorado, for example, ArcGIS offers “NAD 1983 HARN StatePlane Colorado North FIPS 0501″, while the best QGIS can do is “NAD83(HARN) / Colorado North”.  When projection information doesn’t agree exactly, I get [...]

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The Power of Projections

As a spatial professional, projections and geodesy have become second nature to me – often to a point that I take them for granted.  In many cases, the projection becomes like a font in a presentation – if you used Helvetica on page one, you had better use it on page two.  Generally, the consistency [...]

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PostGIS Projection Project

PostGIS is a wonderful suite of tools, which really anchors the open source web mapping toolkit.  However, it doesn’t always integrate well with an ArcGIS desktop environment.  One obvious source of conflict relates to projections – most of ArcGIS’s projections are infinitesimally different than their real world counterparts.  As an example, consider the Washington North [...]

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Revenge of the Map Unit

In my previous post, “When Scale Bars Lie”, I mentioned the dangerous things that can happen when the map unit is not set correctly. As I explained, it can lead to abnormal values, which are often manifested in scale bars.  This week, a friend of mine was using GIS to calculate the areas of some [...]

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When Scale Bars Lie

Occasionally, when knocking up a demo or a quick project in ArcGIS, you may find that your scale bar is lying to you.  In the majority of cases, most GIS users don’t even take notice of their scale bars, much less question the veracity of the distances implied.  However, if you have a projection or [...]

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