Tuesday, November 08, 2005

Fence for home on a farm

Since moving to our farm a little more than a year ago, we've acquired an assortment of animals: geese, chickens, dogs, goats, llamas, etc... Some of the more annoying of these animals are very good at escaping from designated pens and finding their way to our porch and pool and laying waste to vegetation, pooping on our mats, defecating in the pool. sheesh!

It became abundantly clear, at least to me, we need a fence to keep out the riff-raff from our home area. The fence needed to be aesthetic, not too tall to block our view over the top, and inexpensive. We have heard that deer do not like to jump fences they cannot see through, but we didn't want to spend the $ on a pure wood fence. Hence we decided on a wire mesh fence created with treated fir posts.

Here is the design: 5' treated fir posts (3-4" diameter) set 2' in the ground with 3/4" gravel surrounding. The gravel is 3/4" open (meaning there are no smaller pieces than 3/4"-- it does not pack as tight as a 3/4" minus but drains a bit better. We figure the top posts will help secure the fence itself so we don't have to worry too much about packing). The posts are braced on the top with another post that is notched to sit firmly on top of each post. The posts are spaced 10' apart from each other with 5" galvanized spikes holding the tops in place. We will be using a 2"x4" welded mesh wire stapled to the posts and stretched (not too tight!) and held in place with staples. The galvanized mesh is 36" high. When the fence is done, we will be planting vines, shrubs, and some small trees along the fence to create a visual barrier for animals attempting entry or perhaps jumping.

Approximate cost of fence:
100 10' 3-4" treated fir posts .... $700
9 rolls 2x4" welded wire,36" high... $385
~4 tons 3/4" Open gravel ..... $40
5" galvanized spikes .... $65
plants, etc.... $200

Many thanks to the Tilt-Reuben Family who helped us start this project a couple of weeks ago. I'll post some pictures in a few weeks when we hope to have this project completed!

Thursday, September 15, 2005

Overlaying Mercator Projected WMS Layers on GoogleMaps

One troubling aspect of the GoogleMaps API is their use of the Mercator Projection. This projection is not so bad for looking at large-scale maps (zoomed in), but is awful for visualizing small-scale (zoomed out) data, due to the large degree of exaggeration of land masses near the poles. Making matters more complicated is the fact that most Mercator Projections deal in meters as units while GoogleMaps delivers content to and from their API in decimal degrees. This makes things difficult especially when layering global WMS layers on GoogleMaps.

The screenshot to the left illustrates what happens when layering country boundaries from a WMS using a geographic projection on top of GoogleMaps. The interesting thing is that if we were to zoom in on a particular portion of the world, the boundaries would line up. This is why many people are able to use a geographic projection with Google Maps if only using for large-scale mapping projects.

OK, so what if we wanted to utilize a Mercator Projection from a WMS service on top of GoogleMaps? Here is how I did it, using Mapserver and Proj4 for the WMS content (details at the mapserver website):

1. Add the appropriate Mercator definition in the Proj4 EPSG file:
# Mercator
<54004> +proj=merc +lat_ts=0 +lon_0=0 +k=1.000000 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m no_defs <>

2. Alter the Mapserver Map File so the Output Projection looks like:

3. Add the epsg:54004 projection definition to the WEB section of the Mapfile like:
"wms_srs" "EPSG:4326 EPSG:54004"

4. Make sure that the layers in the mapfile reflect their native projection.

5. Alter the Javascript in the GMaps_WMSSpec_0.1.js code (references to the source of this code appear in an earlier post). Primarily, I added two functions to convert decimal degrees to Mercator Meters so the GoogleMaps API can call the WMS service in a language it can understand:

// dd2MercMeters Conversion Functions
// from http://search.cpan.org/src/RRWO/GPS-Lowrance-0.31/lib/Geo/Coordinates/MercatorMeters.pm
var MAGIC_NUMBER=6356752.3142;
var DEG2RAD=0.0174532922519943;
var PI=3.14159267;
function dd2MercMetersLng(p_lng) {
return MAGIC_NUMBER*(p_lng*DEG2RAD);
function dd2MercMetersLat(p_lat) {
if (p_lat >= 85) p_lat=85;
if (p_lat <= -85) p_lat=-85;
return MAGIC_NUMBER*Math.log(Math.tan(((p_lat*DEG2RAD)+(PI/2)) /2));

6. Make a couple of alterations in the WMSSpec.prototype.getWMSURL function. Attached here is the whole function as I code it:

WMSSpec.prototype.getWMSURL=function(a,b,c, url, layers, format) {
if (!this.Styles) this.Styles="default";
var ts = this.tileSize;
var ul = this.getLatLng(a*ts,(b+1)*ts, c);
var lr = this.getLatLng((a+1)*ts, b*ts, c);
// user mercator for small scale
if (map.getZoomLevel() > 12) {
var bbox = dd2MercMetersLng(ul.x) + "," + dd2MercMetersLat(ul.y) + "," + dd2MercMetersLng(lr.x)+ "," + dd2MercMetersLat(lr.y);
var l_strSRS="SRS=EPSG:54004";
} else {
var bbox = ul.x + "," + ul.y + "," + lr.x + "," + lr.y;
var l_strSRS="SRS=EPSG:4326";
var url = url + "REQUEST=GetMap&SERVICE=WMS&VERSION=1.1.1&LAYERS=" + layers + "&STYLES=" + this.Styles + "&FORMAT=" + format + "&BGCOLOR=0xFFFFFF&TRANSPARENT=TRUE&"+l_strSRS+"&BBOX=" + bbox + "&WIDTH=" + ts + "&HEIGHT=" + ;
return url;

You'll notice in step #6 above that I don't use Mercator for resolutions below 13. This is because I believe the nifty little javascript re-project is not quite as accurate as it can be (hey-- what do you expect from javascript doing reprojections!). Anyway, reverting to latlong at lower zoom levels will fix this.

Thats it! The resulting map will looksomething like the screenshot below. The lines going across the top of the Russian Federation, by the way, I think are an artifact of drawing country boundaries across the international dateline. Since I'm only using point data for my real application i didn't worry about this. Not sure if this is an issue but I think it crept in because my source data is stored in geographic units.

Tuesday, September 06, 2005

Measuring flow in a small creek using a simple weir

Recently, I constructed a simple weir for measuring flow in Owen's Creek at the point where our ranch-road crosses the creek. There is an old metal pipe, 8' in diameter which the water flows through, under the road. In front of this pipe I laid a rough-cut 2x10 at the base by digging out gravel in the stream bed. On top of this I placed another rough-cut 2x10 with a four foot rectangular notch. By measuring the head (height of water flowing over the notch) one can determine the flow rate of the water in the stream (See table below).

One improvement I want to make is placing a piece of metal in front of the board and up an inch or so from the level of the notch. This will make a thinner area for the water to flow over which gives a more accurate measurement.

There are still some leaks from the bottom of the weir, running about 25gpm (a conservative guess on my part).
Following are some estimates of streamflow using this method, with an estimated accuracy of +/- 30gpm:

  • July 30th, 2005: 400gpm

  • September 4th, 2005: 315gpm

  • September 23rd, 2005: 250gpm

  • October 1, 2005: 577gpm (1 inch rain in last 24 hours after no rain all summer)

  • Assuming irrigation rates of 96-146gpm, I will be leaving a downstream flow of 165-215gpm flow (using September 4th measurements).


    Gauge Reading in InchesGPM

    Tuesday, August 30, 2005

    WMS overlays and GoogleMap Transparency

    Following are some notes on my experiences in working with WMS layers and GoogleMaps API. Am posting these notes here as I spent alot of time testing in the last couple of weeks on this and maybe this will help someone else out. Following is an image which illustrates what i'm talking about:

    These ideas are not definitive, just some general notes about how to get around.

    1) WMS layering on Googlemaps. Some posts reccomended having two map objects overlaying in separate divs. I drove myself nuts getting the maps to sync and things become more problematic when implementing multiple events to trigger different actions. Plus, this was all super slow. Hence, I opted for combining WMS layers into a single map object.

    2) I integrated a combination of Brian Flood's WMS code and Kyle Mulka's suggestions for handling overlays into a single function to handle WMS overlays (can overlay WMS ontop of GoogleMaps data or other WMS layers such as terraserver). I reccomend working with scripts they have posted rather than anything i have done.

    3) When Google handles overlays in the map object the Style sheets force transparent images to become opaque. Hence filled polygons with translucent properties block out everything. Worse, Google doesn't assign unique id's to any of the divs they create to simplify working with divs. To get around this, I pointed my javascript DOM to the div container for my map container and then searched all child nodes for references to my overlay WMS service and then dynamically changed the style properties. Here is the JS function which updates the appropriate JS elements:
    // adjust Opacity
    function adjustOpacity(p_pctOpacity) {
    if (p_pctOpacity < .01) p_pctOpacity=.01 if (p_pctOpacity >1) p_pctOpacity=1;
    // adjust opacity of overlay layers
    var JStopmap=document.getElementById('topmap').firstChild.childNodes;
    // loop all elements (google tiles)
    for (var i=JStopmap.length-1;i>0;i--) {
    // search for string of overlay layer
    if (JStopmap[i].src!=null) {
    // HACK -- berkeleymapper is a unique string of the WMS service i'm calling
    // this can be anything.
    if (JStopmap[i].src.indexOf('berkeleymapper') > 0) {
    // browsers handle opacity differently, these settings should handle most common methods

    4) Finally, I implemented a slider bar with some modifications in my script to link it up with the adjustOpacity function. This is from Mark Wilton-Jones library of JS functions
    There were other slider scripts out there but this one seemed easiest to work with, though i don't care much for the colors and it is a bit involved to change the look of the control.

    Wednesday, August 24, 2005

    Fun with projections

    Two error circles representing point certainty are displayed on the first map in geographic space. This rendering is from my first implementation of BerkeleyMapper which used geographic space for all maps.

    The second map displays these same error radii in the second implementation of BerkeleyMapper which leverages GoogleMaps API. Google uses a Mercator projection, which preserves local shape but has a large degree of exaggeration towards the poles. Both of these samples are technically correct I suppose but I'm not so happy with the vertically exaggerated ellipse from the GoogleMaps API example.

    The vertical exaggeration of the ellipse in the second sample, however, is preferable to the "squishing" affect of geographic space, particularly when underlaying USGS topos (as the map text will appear flattened in northern and southern latitudes).